A Level Computer Science (9618) — MCQs & Practice Questions

A computer is built from electronic switches, each of which has only two reliable states — on and off, or high and low voltage. Binary has exactly two digits, so each digit maps directly onto one switch, making the hardware simple and resistant to electrical noise.

Place values 16, 8, 4, 2, 1: 16 + 8 + 0 + 2 + 0 = 26.

It is far shorter and easier to read — one hex digit replaces four bits, so a 32-bit address takes 8 hex characters instead of 32 binary ones — while converting between the two needs no arithmetic at all.

1. 2 in hex is 0010 in binaryfour bits per hex digit[1]
2. C is 12 in decimal, which is 1100A=10, B=11, C=12, D=13, E=14, F=15[1]
3. Together: 00101100₂concatenate the groups, do not add them[1]
4. Decimal: 32 + 8 + 4 = 44or 2×16 + 12 = 44 directly from hex[1]

2C₁₆ = 00101100₂ = 44₁₀

1. Adds column by column from the right, carrying where a column totals 2[1]
2. Result within 8 bits: 01000010 with a carry out of the leftmost column[1]
3. In decimal 202 + 115 = 317, which exceeds the 8-bit maximum of 255[1]
4. Overflow occurs — the true answer needs 9 bits, so the stored 8-bit result is incorrectthe consequence must be stated[1]

Carry out of the final column; overflow occurs because 317 > 255.

1. 2⁷ = 128 characters[1]
2. Urdu uses a different script with many additional letterforms, and 128 slots are already taken by English letters, digits and punctuation — so Unicode is used insteadnaming Unicode expected[1]
3. 2²⁴ = 16 777 216 coloursaccept "about 16.7 million"[1]
4. Total pixels = 100 × 200 = 20 000[1]
5. Each pixel takes 24 bits = 3 bytes, so 20 000 × 3 = 60 000 bytesconverting bits to bytes is the step most often missed[1]
6. 60 000 / 1024 = 58.6 KBaccept 58.6 or 60 KB if 1000 bytes per KB is stated[1]

(a) 128, too few for a second script (b) 16 777 216 (c) about 58.6 KB

Advantage: it represents over a million characters, covering every writing system including Urdu and Chinese, where ASCII covers only 128. Disadvantage: each character may need up to four bytes rather than one, so files are larger.

2⁸ = 256 colours.

Sample rate is the number of measurements taken per second, in hertz. Sample resolution is the number of bits used to store each measurement. The first controls accuracy in time, the second accuracy in amplitude.

1. Total pixels = 1024 × 768 = 786 432[1]
2. Total bits = 786 432 × 16 = 12 582 912[1]
3. Bytes = 12 582 912 ÷ 8 = 1 572 864dividing by 8 is the step most often missed[1]
4. Kilobytes = 1 572 864 ÷ 1024 = 1536 KB[1]

1536 KB (1.5 MB)

1. Bits = 44 100 × 16 × 30rate × resolution × seconds[1]
2. = 21 168 000 bits[1]
3. Bytes = 2 646 000, so ÷ 1 048 576 = 2.52 MBaccept 2.65 MB using 1 000 000[1]
4. Stereo records two channels, so the file size doubles to about 5 MB[1]

About 2.5 MB mono, doubling to about 5 MB in stereo

1. Reduce the resolution — fewer pixels means a smaller file, at the cost of visible detail[1]
2. Reduce the colour depth — fewer bits per pixel, at the cost of colour accuracy and possible banding in smooth gradientsaccept compression as a third method[1]
3. The file size is width × height × colour depth, and only the depth has changed[1]
4. 24 is three times 8, and the size is directly proportional to the depth, so the file is three times as largethe proportionality is the mark[1]
5. Consecutive codes mean sorting alphabetically is the same as sorting numerically[1]
6. And converting between upper and lower case is a fixed addition or subtraction of 32, both of which would be far harder with arbitrary assignments[1]

(a) fewer pixels or fewer bits per pixel (b) size is proportional to depth, and 24 is 3 × 8 (c) sorting and case conversion become arithmetic

It uses less storage space, and it transmits faster over a network while using less bandwidth — which reduces cost on a metered connection.

Lossless reduces the file size in a way that allows the original to be reconstructed exactly. Lossy discards some data permanently, giving a much smaller file that can never be restored to the original.

Lossy compression alters the data, and in a program every bit matters — changing even one instruction could stop the program running or make it behave incorrectly. Lossy methods rely on human perception tolerating small changes, and a computer executing code has no such tolerance.

1. Runs identified: five R, three G, eight B[1]
2. Encoded as 5R 3G 8B — six items instead of sixteen characters[1]
3. It works poorly when adjacent values rarely repeat, such as in a photograph where neighbouring pixels differ slightly[1]
4. In that case each run has length 1 and the encoding stores a count as well as the value, making the file larger than the originalthe enlargement is the key insight[1]

5R3G8B; it fails on data with little repetition, where it can increase the file size.

1. Magnetic uses magnetised regions on spinning platters — large capacity and cheap per gigabyte[1]
2. Optical uses pits read by laser — portable and cheap, but small capacity and slow[1]
3. Solid-state advantage: no moving parts, so it is much faster, silent and resistant to being dropped[1]
4. Disadvantage: higher cost per gigabyte, and each memory cell tolerates only a limited number of writesthe write-endurance point is the stronger one[1]

Magnetic cheap and large, optical portable, solid-state fast but dearer with limited write endurance.

1. Photographs: lossy compression such as JPEG[1]
2. The reduction is large and the discarded detail is not noticeable in a photograph viewed normally, so the loss costs nothing in practice[1]
3. Database: lossless compression, or none at all[1]
4. Every value must be recoverable exactly — a changed digit in a mark or a date is simply wrong data, with no perceptual tolerance to exploit[1]
5. Lossless compression works by removing repetition, and once the repetition is gone there is nothing further to remove[1]
6. Compressing an already-compressed file usually achieves nothing and can make it slightly larger, since the compression data itself must be storedaccept that repeated lossy compression degrades quality instead[1]

(a) lossy — the loss is imperceptible (b) lossless — every value must be exact (c) once repetition is removed there is nothing left to compress

A LAN covers a small geographical area such as one building and its hardware is usually owned by the organisation. A WAN covers a large area and typically uses third-party communication links.

An IP address identifies a device on a network and can change when the device moves to a different network. A MAC address is fixed in the hardware and uniquely identifies the network adapter itself.

The data is divided into equal-sized blocks. Each packet carries a header with the source and destination addresses and its sequence number, and packets may travel by different routes and are reassembled in order at the destination.

1. A LAN[1]
2. The computers are all within a single building and the school owns the cabling[1]
3. A switch connects devices within the LAN[1]
4. It forwards each frame only to the port of the intended recipient, using MAC addresses[1]
5. A router connects the LAN to another network, usually the internet[1]
6. It forwards packets between networks using IP addresses[1]
7. Advantage: files and printers can be shared[1]
8. Advantage: software and backups can be managed centrally[1]

1. The browser sends the domain name to a DNS server[1]
2. The server looks up the matching IP address[1]
3. The IP address is returned and the browser requests the page from that server[1]
4. HTTPS encrypts the data in transit, HTTP sends it in plain text[1]
5. So HTTPS protects information such as passwords from being read if intercepted[1]
6. To block access to unsuitable or unauthorised sites[1]
7. To inspect incoming traffic and block unauthorised access to the network[1]

1. Faster and more reliable, with less interference[1]
2. Devices can be moved freely and no cabling is needed[1]
3. Use WPA encryption on the wireless network[1]
4. Data sent over the air is scrambled[1]
5. So anyone intercepting the signal cannot read it without the keyaccept MAC filtering or a strong passphrase, explained[1]

The internet is the global infrastructure of interconnected networks that carries data. The world wide web is a collection of pages and resources accessed over that infrastructure — one of several services using it, alongside email and file transfer.

It translates a domain name that people can remember into the IP address the network needs to route data. This also allows a site to move to a different server without its address changing for visitors.

Keeping a user logged in as they move between pages of a site, and remembering preferences such as language, currency or the contents of a shopping basket.

1. The browser extracts the domain name and requests its IP address from a DNS server[1]
2. The DNS server returns the IP address, querying other DNS servers if necessary[1]
3. The browser sends an HTTP request to that address and the server responds with the HTML[1]
4. The browser requests the further resources the page references, then parses the HTML, applies the CSS and renders the page[1]

DNS lookup → HTTP request → server response → further resource requests → parse and render.

1. The data is encrypted in the browser before it is transmitted[1]
2. so anyone intercepting it on the network sees only ciphertext and cannot read the card details[1]
3. A digital certificate issued by a trusted authority supplies the public key and confirms the site's identity[1]
4. The padlock does not guarantee the site is honest — only that the connection is encrypted and the site is genuinely the one named on the certificatea fraudulent site can hold a valid certificate[1]

Encryption in transit plus a certificate proving identity; the padlock says nothing about the site's honesty.

1. A session cookie is held in memory and deleted when the browser closes — used to keep the customer logged in while moving between pages[1]
2. A persistent cookie is stored on disk and survives until it expires — used to remember preferences or a basket between visits[1]
3. A cookie set by an advertiser present on many different sites can be read on each of them[1]
4. so a record of the user's browsing can be assembled across sites without their being aware of itthe cross-site point is the concern[1]
5. A cookie is a small text file, not a program, so it cannot be executed[1]
6. It can only be read by the site that set it, and cannot access other files on the machine — the genuine risk is to privacy, not to the computer[1]

(a) session in memory until the browser closes; persistent on disk until expiry (b) cross-site tracking builds a browsing profile (c) it is data, not a program

RAM is volatile, fast and comparatively small; it holds the programs and data currently in use and loses everything when the power goes. Secondary storage such as an SSD is non-volatile, slower and much larger; it keeps data permanently.

The control unit, which fetches and decodes instructions and sends control signals to the rest of the system; and the arithmetic and logic unit, which performs calculations and logical comparisons.

It holds the address of the next instruction to be fetched. It increments during the fetch stage so that the following cycle picks up the next instruction rather than repeating the current one.

1. Fetch: the address in the program counter is placed on the address bus and the instruction is brought back along the data bus into the instruction registermention of a bus expected[1]
2. Decode: the control unit interprets the instruction to determine the operation and its operands[1]
3. Execute: the ALU carries out the operation[1]
4. Store: the result is written back to a register or to memory, and the cycle repeatsthe repetition should be stated[1]

Fetch, decode, execute, store — repeated for every instruction.

1. The address bus carries the memory address being accessed[1]
2. The data bus carries the actual data or instruction[1]
3. The control bus carries signals such as read and write[1]
4. The address bus is unidirectional — addresses only ever travel from the CPU to memory, whereas data travels both waysthe reason is required[1]

Address = where (one-way), data = what (two-way), control = read or write.

1. Laptop B[1]
2. Four cores can execute four instruction streams at once, so it handles several programs far better than one faster core[1]
3. 16 GB of RAM means far less swapping to disk, and the SSD has no moving parts so it reads much faster than a hard diskeither point accepted[1]
4. Laptop A has 1 TB of storage against 512 GB, so it holds roughly twice as many filescapacity is the only advantage here[1]
5. Clock speed measures cycles per second on ONE core, so it says nothing about how many cores there are[1]
6. It also ignores cache size and the amount of RAM, both of which decide how often the CPU has to wait for data[1]

(a) Laptop B — more cores, far more RAM, and an SSD (b) twice the storage capacity (c) clock speed ignores cores, cache and RAM

A touchscreen. It displays information, which is output, and senses where the user touches, which is input — both functions in one device.

A sensor produces a continuously varying analogue reading, while a microprocessor can only work with digital values. The ADC converts the analogue signal into a number the processor can use.

A moisture sensor in the soil. It measures the water content directly, which is the quantity the system needs to control, so watering can be triggered when the soil dries below a preset level.

1. An inkjet sprays ink droplets; it is cheap to buy and gives good colour quality, but is slow with a high cost per page[1]
2. A laser uses toner and a drum; it is faster with a much lower cost per page, though dearer to buy[1]
3. Recommend the laser printer[1]
4. 2000 pages a month is high volume, so the lower cost per page and greater speed outweigh the higher purchase price, and invoices need no photographic colour qualitythe justification must use the figures given[1]

Laser — high volume makes speed and cost per page decisive, and invoices need no photo quality.

1. A light or laser is shone at the barcode and the reflection is measured — dark bars reflect less light than the light spaces[1]
2. The pattern of reflections is converted into a number, which is looked up in the product database[1]
3. Advantage: it is much faster than typing a long code[1]
4. Advantage: it is far more accurate — no mistyping — and a check digit in the code catches any misread[1]

Reflected light is read as a pattern and converted to a number; faster and more accurate than typing.

1. A proximity or infrared sensor to detect that a vehicle is present at the barrier[1]
2. A barcode or card reader to read the ticket presented by the driveraccept a camera reading the number plate[1]
3. An actuator — a motor — raises and lowers the barrier arm[1]
4. Sensors only measure; they produce readings and make no decisions[1]
5. A microprocessor must compare the ticket against stored valid tickets and check that a vehicle is actually present[1]
6. Only then does it send a signal to the actuator, so the processor is what turns two readings into an action[1]

(a) a proximity sensor and a ticket reader (b) a motor as actuator (c) sensors only measure — the processor compares and decides

RAM is volatile and loses its contents when the power is removed; ROM is non-volatile and retains them. RAM can be written to freely and holds the programs currently running; ROM is not normally written to and holds the start-up instructions.

RAM is volatile, so everything in it is lost when the machine is switched off, and it is comparatively small and expensive. Secondary storage retains data permanently without power and provides far greater capacity at a much lower cost.

It has no moving parts, so it accesses data much faster, uses less power, makes no noise and is far more resistant to damage from being dropped or vibrated.

1. Virtual memory is an area of secondary storage used as though it were additional RAM[1]
2. It is used when the programs being run need more memory than the physical RAM provides[1]
3. Pages of memory not currently in use are written out to disk, freeing real RAM for active data[1]
4. When a page is needed again it is read back and another is written out — a process called paging or swapping[1]

Disk space used as extra RAM, with unused pages swapped out and read back when needed.

1. RAM has filled, so the operating system is using virtual memory and paging to diskthe constant disk activity is the clue[1]
2. Disk access is thousands of times slower than RAM, so every switch between programs now waits on the disk[1]
3. Installing more RAM is the better remedy: it removes the need to page at all[1]
4. Fitting a faster SSD helps but only shortens each swap — it reduces the symptom while leaving the cause in placethe distinction is the mark[1]

RAM is full and the OS is paging. More RAM removes the cause; a faster disk only shortens each delay.

1. Monitoring machine: solid state, for speed and because it has no moving parts to fail[1]
2. Archive: magnetic hard disks, for the very large capacity at low cost per gigabyte[1]
3. Between sites: optical or solid state portable media, as it must be carried and survive handlingeither accepted with justification[1]
4. If it began paging, the machine would slow unpredictably as it waited on disk access[1]
5. In a monitoring system a delayed response could be dangerous, so the timing must stay predictable — which means never depending on virtual memorythe safety point is the mark[1]
6. ROM holds the start-up instructions that run before any disk is read, so the machine can find and load the operating system — without it nothing could boot at all[1]

(a) SSD, magnetic archive, portable media (b) paging would make response times unpredictable, which is unsafe here (c) ROM boots the machine before any disk is available

Inputs 00 → 1, 01 → 1, 10 → 1, 11 → 0. NAND is the inverse of AND, so it outputs 0 only when both inputs are 1.

(A · B)′ = A′ + B′ and (A + B)′ = A′ · B′. Complementing a whole expression swaps AND for OR and complements each variable.

Because any logic function whatsoever can be built using copies of just one of them. NOT, AND, OR and every larger circuit can be constructed from NAND alone, or from NOR alone, which is why chip manufacturers can make a whole processor from one repeated gate design.

1. Four rows for two inputs, with columns for A′ and B′[1]
2. A=0,B=0 → 0; A=0,B=1 → 1A′·B is 1 in the second row[1]
3. A=1,B=0 → 1; A=1,B=1 → 0[1]
4. Output is 1 exactly when the inputs differ, so this is an XOR gatethe identification is the point of the question[1]

The table matches XOR: output 1 when A and B differ.

1. Expand: A·A + A·B′ + B·A + B·B′distributive law[1]
2. A·A = A (idempotent) and B·B′ = 0 (complement)[1]
3. X = A + A·B′ + A·Bthe zero term disappears[1]
4. By absorption A + A·anything = A, so X = Anaming the law is required by the question[1]

X = A

1. 0+0 → Sum 0, Carry 0; 0+1 → Sum 1, Carry 0[1]
2. 1+0 → Sum 1, Carry 0; 1+1 → Sum 0, Carry 1the last row is the whole point — 1+1 in binary is 10[1]
3. Sum is produced by an XOR gateoutput 1 when the inputs differ[1]
4. Carry is produced by an AND gateoutput 1 only when both are 1[1]
5. A half adder has only two inputs, so it has nowhere to accept the carry produced by the previous column[1]
6. A full adder is needed instead, taking three inputs — A, B and carry-innaming the full adder is expected[1]

(b) XOR for Sum, AND for Carry (c) it has no carry-in input, so a full adder is required

In the von Neumann architecture, program instructions and data are held in the same memory and travel over the same bus. The limitation is the von Neumann bottleneck: instructions and data cannot be transferred simultaneously, so the processor is often left waiting for memory.

The address in the PC is copied to the MAR. The PC is then incremented. The contents of the memory location addressed by the MAR are read into the MDR, and from there the instruction is copied into the CIR ready for decoding.

A jump instruction writes a new address into the PC during execution. If the increment happened afterwards, that new address would be increased by one and the jump would land at the wrong instruction. Incrementing during the fetch means a jump can simply overwrite the PC and execution continues correctly.

1. (a) The address bus is unidirectional; the data bus is bidirectional.Both directions must be stated.[1]
2. The address bus is one-way because only the processor generates addresses — memory never sends one back.The reason is the mark, not the direction alone.[1]
3. (b) Cache is faster than main memory and holds recently or frequently used data, so more requests are met without a slow main-memory access.The speed difference must be mentioned.[1]
4. This reduces the time the processor spends idle waiting for data.Linking to the bottleneck earns the second mark.[1]
5. (c) The program is single-threaded, so the work is not divided between cores and only one core is used.The software, not the hardware, is the limitation.[1]
6. (d) Advantage: several isolated systems on one machine, using hardware efficiently. Disadvantage: slower than running directly on hardware, because of the extra software layer.One of each is required.[1]

(a) address one-way, data two-way; (b) fewer slow memory accesses; (c) single-threaded; (d) isolation vs overhead

SIMD applies a single instruction to multiple data items at the same time — used by graphics processors operating on many pixels identically. MIMD executes different instructions on different data simultaneously — used by multi-core processors running separate tasks.

Assembly language uses short mnemonics in place of binary machine code, with one instruction corresponding to one machine instruction. Because the relationship is one-to-one, translation is a matter of substituting each mnemonic for its opcode — a much simpler task than compiling a high-level language, where one statement may become many machine instructions.

LDM #20 uses immediate addressing: the value 20 itself is loaded into the accumulator. LDD 20 uses direct addressing: the contents of memory address 20 are loaded, which could be any value.

1. (a) 10110110 AND 00001111 = 00000110The mask keeps the lower four bits and clears the upper four.[1]
2. (b) 10110110 OR 01000000 = 11110110OR sets bit 6; every other bit is unchanged because the mask has 0 there.[1]
3. (c) 10110110 XOR 11111111 = 01001001XOR with all ones inverts every bit — a one's complement.[1]
4. (d) 10110110 shifted left 1 = 01101100A zero enters at the right and the leftmost bit is lost, so the value does not simply double here.[1]

(a) 00000110 (b) 11110110 (c) 01001001 (d) 01101100

1. (a) AND with 00000100AND isolates the bit of interest and clears all others.[1]
2. If the result is non-zero the flag is set; if zero it is clear.The test on the result is part of the answer.[1]
3. (b) OR with 00000100OR forces a 1 into that position without disturbing the others.[1]
4. (c) AND with 11111011The complement of the mask — a 0 in the target position clears it, 1s elsewhere preserve.[1]
5. (d) A logical shift brings in a 0 at the left, which would overwrite the sign bit and turn a negative number positive.The sign bit is the crux.[1]
6. An arithmetic shift replicates the sign bit instead, so the sign is preserved and the division gives the correct signed result.Both halves — what goes wrong and what is done instead — are needed.[1]

(a) AND 00000100, non-zero means set; (b) OR 00000100; (c) AND 11111011; (d) preserves the sign bit

It multiplies the value by 2³ = 8. The result would be wrong if any bits shifted off the left-hand end were 1s, since those bits are lost — the value would then have overflowed the available word length.

The core of the OS, permanently resident in memory, which manages the CPU, memory and devices and controls access to them. Other parts of the OS such as the user interface can be restarted or replaced; the kernel cannot.

Through time-slicing: the operating system gives each program a few milliseconds of CPU time in turn and switches between them far faster than a person can perceive. Only one instruction is ever executing, so the simultaneity is an illusion.

Any program that runs inherits the permissions of the account that started it. Malware launched from an administrator account can therefore alter system files and install itself permanently, whereas the same malware run from a standard account is confined to that user's own files.

1. Process management — scheduling which program uses the CPU and for how long[1]
2. Memory management — allocating RAM to each program and keeping them separate[1]
3. File management — organising storage into files and folders and controlling access to them[1]
4. Device management — communicating with hardware through drivers; also accept security and user accounts[1]

Process, memory, file and device management — plus security and the user interface.

1. RAM has filled, so the OS is moving pages of memory out to disk — paging or swappingthe constant disk light is the clue[1]
2. Disk access is thousands of times slower than RAM, so every switch between programs now waits on the diskthe speed gap is the explanation[1]
3. Solution: close unused applications and reduce the number of programs starting automatically[1]
4. Solution: install more RAM, which removes the need to page rather than merely making paging fasteran SSD helps but does not fix the cause[1]

RAM is full and the OS is paging to disk. Close programs, or add RAM.

1. Office machines: a single-user multi-tasking operating system, since one person at a time runs several applications togetheraccept multi-user for shared machines[1]
2. Patient monitor: a real-time operating system[1]
3. Real-time guarantees that a response occurs within a fixed maximum time, not merely quickly on averagethe guarantee is the distinguishing feature[1]
4. For a patient monitor an alarm raised late is as dangerous as no alarm, so a predictable worst case matters more than average speed[1]
5. File permissions grant read, write and execute rights separately to the owner, a group and others[1]
6. So clinical staff can be given read access to records while administrative staff cannot open them at all, and only authorised users may alter themthe example must fit the hospital[1]

(a) multi-tasking for the office, real-time for the monitor (b) a guaranteed maximum response time (c) separate read and write rights per group

A compiler translates the whole program before it runs and produces an executable file, whereas an interpreter translates and executes one statement at a time and produces no file. A compiler also reports all errors together at the end, while an interpreter halts at the first error it meets.

Lexical analysis breaks the source into tokens, removes comments and whitespace, and builds the symbol table. Syntax analysis checks that the sequence of tokens obeys the grammar of the language, building a parse tree and reporting errors such as a missing bracket.

Semantic analysis. The statement is grammatically well-formed, so it passes syntax analysis, but it is meaningless because a string cannot be added to an integer and assigned to an integer variable — a type mismatch.

1. (a) An interpreter reports the first error immediately and lets a change be tested without a full rebuild, which suits development.The fast feedback loop is the point.[1]
2. A compiled executable runs faster and can be distributed without the source code, which suits release.Speed and source privacy are both valid.[1]
3. (b) Optimisation improves the generated machine code so it runs faster or takes less space.Either faster or smaller is acceptable.[1]
4. For example, removing code that can never execute, or keeping a frequently used value in a register instead of memory.A concrete example is required for the mark.[1]
5. (c) Bytecode is an intermediate form produced by the compiler, executed by a virtual machine rather than directly by the processor.The intermediate nature must be stated.[1]
6. It is portable: the same compiled file runs on any platform with a suitable virtual machine, without recompiling.Portability is the advantage being tested.[1]

(a) fast feedback vs speed and privacy; (b) faster or smaller code; (c) portable intermediate form

In assembly language each mnemonic corresponds to exactly one machine instruction, so translation is largely a matter of substituting opcodes and resolving addresses. A compiler must translate high-level statements that may each become many machine instructions, and must also perform syntax, semantic and optimisation work.

A virus attaches itself to a host file and requires a user to run that file in order to spread. A worm is self-replicating and spreads across a network by itself, needing no user action at all.

An attacker tries every possible password in turn until one works. The most effective measure is to lock the account after a small number of failed attempts, which makes exhaustive guessing impossible however fast the attacker is.

It examines data entering or leaving a network and blocks anything not permitted by its rules, preventing unauthorised access and filtering out unwanted traffic.

1. Phishing sends a fraudulent message imitating a trusted organisation, containing a link to a fake site the victim must click[1]
2. Defence: check the sender and the true destination of any link, and never log in through a link — type the address yourself[1]
3. Pharming uses malicious code to alter DNS settings so that typing the correct address still leads to a fake siteno link is clicked[1]
4. Defence: anti-malware to prevent the DNS settings being altered, and checking the HTTPS certificate matches the site you intendedtyping the address does not help here[1]

Phishing needs a click; pharming redirects a correctly typed address. Different attacks, different defences.

1. Firewall — filters incoming and outgoing traffic, preventing unauthorised access from outside[1]
2. Anti-malware with automatic updates — detects and removes viruses, worms and spyware, and updates close known vulnerabilities[1]
3. Access levels — each user sees only what they need, so a compromised student account cannot reach staff records[1]
4. Two-factor authentication or strong password policy with lockout — protects against brute force and stolen passwordsaccept staff training against social engineering[1]

Firewall, anti-malware with updates, access levels, and strong authentication — each matched to its threat.

1. The website incident is a DDoS attack — the server is flooded with requests from many machines until it cannot respond to genuine users[1]
2. The password incident is social engineering — the attacker manipulated a person into revealing the password, with no technical attack at all[1]
3. A firewall filters network traffic against a set of rules[1]
4. The employee volunteered the password over the telephone, which never touched the network — no traffic rule could have detected itthe reason is the mark[1]
5. Against DDoS: traffic filtering at the firewall, rate limiting, or a service that absorbs and distributes the load[1]
6. Against social engineering: staff training and a strict procedure that credentials are never given out over the phone, with any such request verified by calling back on a known number[1]

(a) DDoS and social engineering (b) the password never crossed the network (c) traffic filtering, and training with a call-back verification policy

It scrambles the data using a key so that it becomes meaningless to anyone who intercepts it. Only someone with the correct key can decrypt it back into readable form.

No. The data can still be intercepted, copied or deleted exactly as before. What encryption prevents is the interceptor understanding what they have taken, because without the key the ciphertext is unreadable.

Each extra bit doubles the number of possible keys, so a brute force attack — trying every key in turn — takes far longer. A key short enough to be tried exhaustively offers no real protection.

1. Symmetric uses the same key to encrypt and decrypt[1]
2. Advantage: it is much faster, so it suits large volumes of data[1]
3. Asymmetric uses a public key to encrypt and a matching private key to decrypt[1]
4. Advantage: no secret key needs to be exchanged, which removes the problem of getting the key to the recipient safelythe key distribution problem is the point[1]

Symmetric: one key, fast. Asymmetric: a public/private pair, no shared secret needed.

1. A public algorithm can be examined by cryptographers worldwide[1]
2. so weaknesses are found and corrected, rather than remaining hidden until an attacker discovers one[1]
3. Security rests entirely on the key, which remains secret — knowing the method does not read the message[1]
4. A secret algorithm has been checked by very few people, and once leaked offers no protection at all — "security through obscurity" is not security[1]

Public scrutiny finds weaknesses; secrecy rests on the key alone, not on the method.

1. The data is encrypted in the browser before it is sent, so anything intercepted on the way is ciphertext[1]
2. Only the shop's server holds the key needed to decrypt it, so an interceptor sees nothing usable[1]
3. Asymmetric encryption is used first, to exchange a symmetric key safely without any prior shared secret[1]
4. Then symmetric encryption is used for the actual data, because it is far faster for the volume involvedthe combination is the mark[1]
5. Encryption does not protect the data once it has been decrypted at the destination[1]
6. If the shop's database is later breached, or if the customer was tricked into sending details to a fake site in the first place, encryption offers no protection — it secured the journey, not the endpointsaccept phishing as the example[1]

(a) encrypted before sending, decryptable only by the server (b) asymmetric to exchange the key, symmetric for speed (c) it protects the journey, not the endpoints — a breached database or a fake site defeats it

One bit of each byte is set so that the total number of 1s is even. The receiver counts the 1s in the byte it received; if the total is odd, at least one bit has been corrupted in transmission.

If two bits flip in the same byte, the parity is restored and the error is not detected. It also cannot identify which bit is wrong, so the data must be resent rather than repaired.

An extra digit calculated from the others and appended to a code such as an ISBN, barcode or account number, so that a data entry error can be detected immediately when the code is typed in.

1. Each byte carries a parity bit, so a corrupted byte fails its own row check[1]
2. An extra parity byte is sent whose bits give the parity of each column across the block[1]
3. A single flipped bit causes exactly one row and exactly one column to fail their checks[1]
4. The bit at the intersection of that row and column is the faulty one, so it can simply be flipped back — no resend is neededthe intersection is the key idea[1]

The failing row and failing column intersect at the corrupted bit, which is then flipped back.

1. The receiver checks each packet using an error-detection method such as a checksum[1]
2. It sends back a positive acknowledgement if the packet is intact, or a negative one if it is corrupted, prompting a resend[1]
3. The sender starts a timer when it transmits each packet[1]
4. If no acknowledgement arrives before the timeout, the packet is assumed lost and resent — which covers a packet that never arrived, since the receiver had nothing to checkthe timeout is the mark most often missed[1]

Acknowledgements handle corrupted packets; a timeout handles ones that never arrive.

1. Attenuation — the signal weakens over the distance until a bit is misread[1]
2. Interference from nearby electrical equipment, which a hospital has a great deal ofaccept skew if parallel[1]
3. A checksum on each packet, since it detects most corruption across the whole block rather than one byte at a timeaccept parity block with justification[1]
4. Patient data must be exactly correct — a single wrong digit in a dosage could cause harm — so a strong check is justified even at the cost of extra datathe justification must fit the context[1]
5. Detection only tells you the data is wrong; it does not deliver the correct data[1]
6. ARQ must be added so that any packet failing the check, or failing to arrive at all, is automatically requested again until it is received intact[1]

(a) attenuation and interference (b) a checksum, because the data must be exact (c) detection must be paired with ARQ so faulty packets are resent

A legal issue concerns what the law permits — for example copying licensed software without paying, which breaches copyright. An ethical issue concerns what is right regardless of the law — for example a company legally selling users' browsing histories to advertisers without making that clear.

Phishing is an attempt to obtain personal information such as passwords by sending a message that imitates a trusted organisation. Recognise it by checking the sender's actual address and the real destination of any link, and by treating urgency — "your account will be closed today" — as a warning sign rather than a reason to hurry.

The digital divide is the gap between people who have reliable access to computers and the internet and those who do not — whether because of cost, location, or lack of skills. It matters because as services move online, those without access are excluded from them.

1. Collect only the data needed for a clearly stated purpose (data minimisation)[1]
2. Use it only for that stated purpose, and obtain informed consent[1]
3. Keep it accurate, secure and protected against unauthorised access[1]
4. Retain it no longer than necessary, and allow individuals to see and correct what is held about them[1]

Minimise, use only for the stated purpose, keep secure and accurate, and delete when done.

1. Ransomware is malware that encrypts the victim's files so they cannot be opened[1]
2. It then demands payment in exchange for the decryption key, with no guarantee the key will be supplied[1]
3. It encrypts everything it can reach, including attached backup drives and automatically synchronised cloud foldersthis is why ordinary backups fail[1]
4. A backup kept disconnected cannot be reached and therefore cannot be encrypted, so the files can be restored without paying[1]

It encrypts files and demands payment; only a disconnected backup is out of its reach.

1. It may well be legal, since the practice is disclosed in the terms the user agreed to[1]
2. But it is ethically questionable, because burying it in clause 47 of a document nobody reads means users have not meaningfully agreedboth categories must be addressed[1]
3. Informed consent requires the user to be told clearly, in plain language and before agreeing, exactly what is collected and who it is shared with[1]
4. It must be a genuine choice — the service should not be withheld unless the user agrees to data collection unrelated to providing it[1]
5. Harm: a continuous location history reveals home and workplace addresses, daily routines and religious or medical visits, enabling stalking or burglary[1]
6. Harm: it could be used for identity theft, blackmail, or discrimination by employers or insurersany second concrete harm[1]

(a) probably legal but ethically poor (b) plain language, in advance, a genuine choice (c) stalking or burglary from routine data; blackmail or discrimination

A primary key is a field, or combination of fields, that uniquely identifies each record in a table. It must be unique across all records and must never be empty. It should also never change.

A field in one table that holds the primary key value of another table, creating the link between them. For example, ClassID stored in the STUDENT table refers to the primary key of the CLASS table.

Names are not unique — two students may share one — and they can change, which would break every foreign key referring to that record. A primary key should be a meaningless, stable identifier such as a roll number.

1. Data integrity — validation rules and constraints reject invalid data[1]
2. Security — access permissions per user, per table, or per field[1]
3. Concurrency control — simultaneous updates by different users cannot corrupt the data[1]
4. Backup and recovery, and a query language for answering unanticipated questionseither earns the mark[1]

Integrity, security, concurrency control, backup and recovery, and a query language.

1. Relationship identified as one to many, from instructor to student[1]
2. INSTRUCTOR(InstructorID, Name, Phone)primary key underlined or stated[1]
3. STUDENT(StudentID, Name, Address, InstructorID)[1]
4. InstructorID in STUDENT is a foreign key referring to INSTRUCTOR — the key goes on the "many" sideplacing it on the wrong side is the standard error[1]

Two tables, with InstructorID as a foreign key in STUDENT.

1. Many to many — a student takes many courses and a course has many students[1]
2. A field holds a single value, so neither table can hold a list of the other; a third table is requiredthe reason, not just the assertion[1]
3. STUDENT(StudentID, Name) and COURSE(CourseID, Title)[1]
4. ENROLMENT(StudentID, CourseID, Grade) with a composite primary key, both fields also being foreign keysaccept a separate EnrolmentID[1]
5. The grade belongs in ENROLMENT[1]
6. Because it is a property of that particular student's enrolment in that particular course — not of the student, who has many grades, nor of the course, which awards manythe reasoning is the mark[1]

(a) many to many — a field cannot hold a list (b) STUDENT, COURSE and an ENROLMENT linking table (c) the grade goes in ENROLMENT, since it belongs to the relationship

Conceptual design — identifying entities, attributes and relationships. Logical design — converting these to tables with keys and normalising them. Physical design — choosing data types, indexes and storage for the particular DBMS.

A noun with properties of its own that must be stored is an entity and becomes a table. A noun that is simply a single property of something else is an attribute and becomes a field. "Address" is usually an attribute of a customer, but becomes an entity if the system must store a district, postcode and access notes for each one.

The number of records on each side of a relationship that may be associated with one record on the other — written 1:1, 1:M or M:N. It determines the table structure, since a 1:M becomes a foreign key while an M:N requires an additional linking table.

1. Entities: TITLE and PUBLISHERbookshop is the system, not an entity[1]
2. Relationship: one publisher supplies many titles, and each title has one publisher — 1:Mstated in both directions[1]
3. PUBLISHER(PublisherID, Name, Address)[1]
4. TITLE(ISBN, Title, Price, PublisherID) — the foreign key on the many sideISBN is a natural primary key here[1]

TITLE and PUBLISHER, in a 1:M relationship, with PublisherID as a foreign key in TITLE.

1. A field holds a single value, so it cannot store a list of related recordsthe fundamental reason[1]
2. Putting a foreign key on either side would allow only one match, which contradicts "many"[1]
3. A third linking table is created, containing the primary keys of both entities as foreign keys[1]
4. This converts the M:N into two 1:M relationships, each of which tables can express; the linking table also holds any attribute of the relationship itself[1]

A field holds one value, so a linking table with both foreign keys is required, giving two 1:M relationships.

1. Entities: MEMBER, CLASS, INSTRUCTORgym is the system itself[1]
2. INSTRUCTOR to CLASS is 1:M — one instructor teaches many classes, each class has one instructor[1]
3. MEMBER to CLASS is M:N — a member attends many classes and a class has many membersstated in both directions[1]
4. INSTRUCTOR(InstructorID, Name) and CLASS(ClassID, ClassName, Time, InstructorID)InstructorID is the foreign key on the many side[1]
5. MEMBER(MemberID, Name, JoinDate)[1]
6. ATTENDANCE(MemberID, ClassID, Date) — a linking table with a composite key, resolving the M:NDate belongs here, being a property of that attendance[1]

(a) MEMBER, CLASS, INSTRUCTOR (b) instructor–class 1:M, member–class M:N (c) four tables, with ATTENDANCE linking members to classes

Validation is an automatic check that entered data is reasonable — within range, of the right type, in the right format. Verification checks that the data was entered correctly, for example by double entry or by reading it back. A date typed as 1995 instead of 1985 passes validation but fails verification.

The rule that a foreign key must either be empty or match an existing primary key in the related table. It prevents orphaned records — a student enrolled in a class that does not exist, or left pointing at a class after it is deleted.

It contains no repeating groups — every cell holds a single, indivisible value, and there are no columns such as Subject1, Subject2, Subject3. Every record is also uniquely identifiable by a primary key.

1. Insertion anomaly — a fact cannot be recorded without another. A new teacher cannot be entered until a student enrols with them[1]
2. Deletion anomaly — removing one fact destroys another. Deleting the last enrolment erases the teacher's phone number[1]
3. Update anomaly — one change must be made in many rows[1]
4. Missing one row leaves the database holding two different values for the same fact, so no report can be trustedthe consequence is the fourth mark[1]

Insertion, deletion and update anomalies, all caused by storing a fact in more than one row.

1. ClassTeacher depends on ClassID, not on the primary key RollNo[1]
2. RollNo → ClassID → ClassTeacher is a transitive dependency through a non-key field, which 3NF forbidsnaming it is required[1]
3. Split into STUDENT(RollNo, Name, ClassID)ClassID remains as a foreign key[1]
4. and CLASS(ClassID, ClassTeacher) — the teacher is now stored once per class[1]

A transitive dependency; split into STUDENT and CLASS tables.

1. MemberName is repeated on every loan by that member, and BookTitle and Author on every loan of that book — redundancy[1]
2. An update anomaly follows: correcting a member's name means editing every one of their loan records, and missing one leaves contradictory dataaccept insertion: a new book cannot be recorded until it is borrowed[1]
3. MEMBER(MemberID, MemberName)[1]
4. BOOK(BookISBN, BookTitle, Author)[1]
5. LOAN(LoanID, MemberID, BookISBN, DateOut) — both foreign keys, and DateOut belongs to the loan itself[1]
6. Benefit: each fact is stored once, so updates are single edits and no contradiction is possible. Cost: retrieving a full loan report now requires joining three tables, which is more complex and slower to queryboth halves needed for the mark[1]

(a) redundancy and update anomalies (b) MEMBER, BOOK and LOAN (c) single-point updates, at the cost of joins

DDL defines or alters the structure of the database — for example CREATE TABLE. DML operates on the data held in the rows — for example SELECT or INSERT.

WHERE is applied to individual rows before grouping, so at that point no counting has taken place and there is nothing to compare. A condition on an aggregate must appear in a HAVING clause, which is applied to the groups after aggregation.

CREATE TABLE Book (ISBN VARCHAR(13) PRIMARY KEY, Title VARCHAR(100) NOT NULL, YearPublished INTEGER);

1. (a) SELECT Name FROM Employee WHERE Salary > 50000 ORDER BY Name;A condition on a plain column belongs in WHERE.[1]
2. ORDER BY Name gives alphabetical order (ASC is the default).The ordering clause is a separate mark.[1]
3. (b) SELECT Department.DeptName, AVG(Salary) FROM Employee JOIN Department ON Employee.DeptID = Department.DeptIDThe join is needed because the two pieces of information sit in different tables.[1]
4. GROUP BY Department.DeptName;Grouping is what produces one row per department.[1]
5. (c) Add HAVING AVG(Salary) > 40000 after the GROUP BY clause.An aggregate condition, so HAVING rather than WHERE.[1]
6. (d) Every row in the table is updated, because there is no condition restricting which rows match.The consequence must be stated as affecting all rows.[1]

(a) WHERE + ORDER BY; (b) JOIN + GROUP BY; (c) HAVING; (d) every row is changed

DELETE FROM Student; removes every row but leaves the table structure in place, so it can still be used. DROP TABLE Student; removes the table itself — structure and data — so it no longer exists.

A finite sequence of unambiguous instructions that solves a problem or completes a task, with clearly defined inputs and outputs.

Abstraction is removing the details that are not relevant to the problem being solved, so that the essential structure is easier to work with. A metro map shows the order of the stations and the connections between lines, but omits real distances and street layout — which makes it more useful for planning a journey, not less.

A flowchart is graphical, using standard shapes and arrows, and shows the flow of control visually. Pseudocode is textual, resembles program code and is quicker to write and to translate into a real language. Flowcharts become unwieldy for long algorithms, where pseudocode stays readable.

1. INPUT mark[1]
2. Validation: WHILE mark < 0 OR mark > 100 → OUTPUT "Invalid" and INPUT mark againa loop, not a single if — the user may enter a bad value twice[1]
3. IF mark >= 40 THEN OUTPUT "Pass"the >= is required; > alone fails a mark of exactly 40[1]
4. ELSE OUTPUT "Fail" ENDIF[1]

Input with a validation loop, then IF mark >= 40 output Pass ELSE output Fail.

1. Columns for n, i, total, and outputone column per variable[1]
2. After pass 1: i = 2, total = 1; after pass 2: i = 3, total = 3[1]
3. After pass 3: i = 4, total = 6; after pass 4: i = 5, total = 10[1]
4. The loop ends because 5 > 4; output is 10the algorithm sums 1 to n[1]

Output 10 — the algorithm computes 1 + 2 + 3 + 4.

1. Break the system into separate sub-problems, such as: record one student's attendance; store a day's records; calculate one student's monthly totalat least two named sub-problems[1]
2. And: produce the report; handle login. Each is small enough to be designed and tested on its ownthe reason for decomposing is part of the answer[1]
3. Abstraction: store only what the task needs — a student's roll number and present/absent flag — and ignore irrelevant details such as their address or hobbies[1]
4. The unnecessary detail would make the system larger and slower without improving the report[1]
5. Boundary data tests the values at the edges of what is acceptable, which is where most errors occur[1]
6. Example: a month with 31 days, or a class with exactly one student, or an attendance percentage of exactly 75% if that is the pass thresholdany sensible boundary value[1]

(a) split into recording, storing, calculating and reporting (b) store only roll number and present/absent (c) edges are where errors hide — e.g. exactly 75% attendance

The data must already be sorted. The algorithm discards half the list at each step by comparing with the middle item, which is only valid if the order is known.

A measure of how the number of operations grows as the size of the input grows, written in big-O notation. It describes the trend, not the exact running time on a particular machine.

About 10, because 2¹⁰ = 1024. Each comparison halves the remaining list, so the maximum is log₂ n rounded up.

1. After pass 1: 3, 5, 1, 8[1]
2. After pass 2: 3, 1, 5, 8[1]
3. After pass 3: 1, 3, 5, 8[1]
4. Three passes; a fourth confirms no swaps[1]
5. A flag records whether any swap was made; if a whole pass makes none, the list is sorted[1]
6. O(n²)[1]
7. Doubling the data quadruples the work, so it becomes impractically slow[1]

1. Initialises low = 1 and high = 5000[1]
2. Loops while low <= high[1]
3. Calculates mid = (low + high) DIV 2[1]
4. Compares and returns mid if the name matches[1]
5. Otherwise sets high = mid − 1 or low = mid + 1 correctly[1]
6. Returns a value such as −1 or FALSE once low > high[1]
7. Binary search: at most 13 comparisons, since 2¹³ = 8192[1]
8. Linear search: up to 5000, so binary search is dramatically faster[1]

1. Merge sort or quicksort (average case)[1]
2. The log n factor grows very slowly compared with the extra factor of n[1]
3. So the gap between the two widens rapidly as n increases[1]
4. Doubling n slightly more than doubles n log n[1]
5. So roughly 4.2 to 4.5 seconds, a little over doubleaccept "just over 4 seconds" with reasoning[1]

A field is a single item of data, such as a student's name. A record is the complete set of fields describing one entity — all the details of one student. A table is a collection of records of the same type.

A numeric type would discard a leading zero, could not store spaces or dashes, and might round the value. No arithmetic is ever performed on a telephone number, so nothing is gained by making it numeric.

Redundancy is the same data stored in more than one place. Inconsistency is what happens when one copy is updated and another is not, so the system holds conflicting values. Redundancy wastes storage, which is cheap; inconsistency means no report can be trusted, because there is no way to tell which copy is correct.

1. Redundancy — the same data is duplicated across several files[1]
2. Inconsistency — updating one copy and not another leaves conflicting values[1]
3. Data dependence — the file layout is embedded in each program, so a change to the structure means editing every program[1]
4. Poor security and sharing — access is all-or-nothing per file; also accept difficulty answering unanticipated queries[1]

Redundancy, inconsistency, data dependence, and poor security or difficult queries.

1. ProductCode — text, since it may contain letters and leading zeros and is never used in arithmetic[1]
2. Quantity — integer, since you cannot sell a fraction of an item and it is counted[1]
3. Price — decimal or currency, since it has a fractional part and must keep exact precisiona float can introduce rounding errors in money[1]
4. DateSold — date, so that date arithmetic and sorting work correctly, which they would not on text[1]

Text, integer, currency, date — each justified by what will be done with it.

1. Redundancy — each patient's name, address and identifier are stored three times[1]
2. Inconsistency — a change of address entered at reception does not reach pharmacy or billing, so the systems disagree and nobody knows which is rightin a hospital this can affect care, not just correspondence[1]
3. A database stores each patient record once, so there is only one copy to update and no second copy to disagree with[1]
4. All three departments read the same record, so a change is visible everywhere immediately[1]
5. Permissions can be set per table or per field, so pharmacy sees prescriptions and billing does notbetter than all-or-nothing file access[1]
6. A database does not fix incorrect data entry — if the address is typed wrongly it is now wrong everywhere at once, so validation at entry is still essentialthe honest limitation[1]

(a) redundancy and inconsistency (b) one stored copy, seen by all, with per-table permissions (c) it cannot prevent wrong data being entered — and now the error is everywhere

Advantage: any element can be reached directly from its index in constant time. Disadvantage: inserting at the front requires every later element to be shifted.

A stack is last-in-first-out: items are added and removed at the same end. A queue is first-in-first-out: items are added at one end and removed from the other.

On a push the pointer is incremented and the new item is written at that position. On a pop the item at the pointer is read and the pointer is decremented. The pointer must be checked against the array bounds first.

1. Jobs must be printed in the order they were submitted[1]
2. A queue removes items in the same order they were added, which is first-in-first-out[1]
3. In a linear queue the front pointer moves forward and the space behind it is wasted[1]
4. A circular queue wraps the rear pointer round to the start of the array[1]
5. So the freed space is reused and the array does not appear full while space remains[1]
6. That the queue is not full[1]

1. 50 at the root with 30 on its left and 70 on its right[1]
2. 20 and 40 as the children of 30[1]
3. 60 as the left child of 70[1]
4. Visits left subtree, then node, then right subtree[1]
5. 20, 30, 40, 50, 60, 70[1]
6. The values in ascending order[1]
7. It can therefore be used to sort the data, or to output it in order without re-sorting[1]

(b) 20, 30, 40, 50, 60, 70

1. Traverse from the head until the node after which Chand belongs is found, here Bilal[1]
2. Create the new node and set its pointer to whatever Bilal was pointing at, that is Danish[1]
3. Set Bilal's pointer to the new nodethis order matters or the rest of the list is lost[1]
4. The nodes are not stored in consecutive memory locations[1]
5. Order is held by the pointers, so changing two pointers is enough[1]

Variables are held in RAM, which is volatile — their contents are lost the moment the program ends or the power fails. A file is stored on secondary storage, so the data persists and can be read by the same program on another day or by a different program entirely.

It returns NULL. Using a NULL file pointer in any subsequent read or write causes the program to crash, so the check turns an unexplained failure into a clear message such as "cannot open file".

"w" erases the entire contents of an existing file the moment it is opened, then writes from the beginning. "a" preserves the contents and adds new data at the end. Both create the file if it does not exist.

1. FILE *fp; int n; fp = fopen("data.txt", "r");[1]
2. if (fp == NULL) { printf("Cannot open file\n"); return 1; }the NULL check is a mark[1]
3. while (fscanf(fp, "%d", &n) == 1) { printf("%d\n", n); }testing the return value, not feof[1]
4. fclose(fp);[1]

Open, check NULL, loop while fscanf returns 1, close.

1. The file has been opened in "w" mode instead of "a"[1]
2. "w" truncates the file to zero length as soon as it is opened, destroying every existing recordthe truncation happens on opening[1]
3. The new record is then written into an empty file, which is why only it remains[1]
4. Correct it by opening with "a", which preserves the contents and appends at the end[1]

The file was opened with "w", which truncates it. Use "a" to append.

1. Opens the file in "r" mode and checks the pointer is not NULL[1]
2. while (fscanf(fp, "%s %d", name, &marks) == 2) {two items requested, two expected back[1]
3. if (marks >= 50) count++; } then prints the count after the loopcount initialised before the loop[1]
4. fclose flushes the output buffer, so data still held in memory is actually written to disk[1]
5. It also releases the file handle, a limited operating-system resource that would otherwise run out[1]
6. feof becomes true only after a read has already failed, so the loop body runs one extra time and the previous values are processed twicetesting the read's return value avoids this[1]

(a) open, check, loop on fscanf == 2 counting passes (b) flushes the buffer and releases the handle (c) feof is set after a failed read, so the last record is processed twice

A variable is a named location in memory holding a value. x = 10 creates the name x and assigns the integer value 10 to it. Assigning again later replaces the value rather than creating a second variable.

Because input() always returns a string, even when the user types digits. Without conversion, + would join the text instead of adding — "5" + "3" gives "53" — and comparisons would compare text rather than numeric value.

= is the assignment operator, storing a value in a variable. == is the comparison operator, testing whether two values are equal and producing True or False.

1. length = float(input("Enter length: "))float rather than int allows decimal measurements[1]
2. width = float(input("Enter width: "))[1]
3. area = length * widtha meaningful variable name is expected[1]
4. print("The area is", area)output must be labelled, not a bare number[1]

Read both values as floats, multiply, and print with a label.

1. (i) 3 — integer division discards the remaindernot 3.5[1]
2. (ii) 1 — the remainder when 7 is divided by 2[1]
3. (iii) 3.5 — / always produces a floatnote the decimal point[1]
4. (iv) 8 — ** is the power operator2 cubed[1]

(i) 3 (ii) 1 (iii) 3.5 (iv) 8

1. Error 1: the inputs are strings, so they are joined rather than addedint() or float() is missing[1]
2. Error 2: operator precedence — a + b / 2 divides b by 2 first, so brackets are needed: (a + b) / 2this error survives even after the type is fixed[1]
3. Corrected input lines using float(input(…))[1]
4. avg = (a + b) / 2 followed by a labelled print[1]
5. Test with normal data, such as 60 and 80, and check the answer is 70a value you can verify by hand[1]
6. Test with boundary data such as 0 and 100, and with erroneous data such as text, to see how it behavesall three kinds of test data[1]

(a) inputs not converted, and missing brackets (b) float(input(…)) and (a+b)/2 (c) normal, boundary and erroneous data

Sequence — statements written one after another, needing no keyword. Selection — if. Iteration — for or while.

When the number of repetitions is not known in advance and depends on a condition evaluated during the loop — for example repeating until the user enters valid input or types "quit". A for loop is used when the count is fixed beforehand.

Five times, with i taking the values 2, 3, 4, 5 and 6. The upper bound 7 is where the sequence stops and is not itself used.

1. for n in range(1, 21): — the upper bound must be 21 to include 20the off-by-one is examined here[1]
2. if n % 3 == 0:the remainder test for divisibility[1]
3. print(n) correctly indented inside the ifindentation is part of the mark[1]
4. Output: 3, 6, 9, 12, 15, 18[1]

A for loop over range(1, 21) with an if n % 3 == 0 test inside.

1. Nothing inside the loop changes count[1]
2. So the condition count > 0 is true on every pass and never becomes false — an infinite loopnaming it as infinite is expected[1]
3. Add count = count - 1 inside the loopaccept count -= 1[1]
4. It must be indented inside the loop; placed outside it would run only once, after the loopthe indentation point is a mark[1]