Error Spotting – Choose the single segment (A–D) that contains a grammatical or spelling error; select ‘‘All correct’’ only if the entire sentence is correct. Sentence: A) They cannot buy B) certain C) medicines which are extremely D) expensive

Introduction / Context:
Not every sentence hides a mistake; some items test your ability to resist overcorrection. The sentence describes a financial constraint on purchasing costly medicines. All four segments are grammatically consistent and idiomatic.

Given Data / Assumptions:

• Modal negative ability: “cannot buy”.
• Determiner “certain” correctly limits the noun “medicines”.
• Relative clause “which are extremely …” modifies “medicines”.
• Predicate adjective “expensive” concludes the clause.

Concept / Approach:
Check agreement, modifier attachment, and idiomatic use. The determiner-noun pairing is fine, the relative clause properly attaches to “medicines,” and degree adverb “extremely” correctly modifies the adjective “expensive.” No punctuation is required inside this simple sentence, and word choice is standard.

Step-by-Step Solution:
1) Validate subject–verb structure: “They … cannot buy” is correct.2) Confirm determiner use: “certain medicines” is idiomatic.3) Inspect the relative clause: “which are extremely expensive” is well-formed.4) Conclude that no segment contains an error; select “All correct.”

Verification / Alternative check:
Rephrase for meaning: “They are unable to purchase some medicines because they are very costly.” This paraphrase preserves structure and confirms correctness.

Why Other Options Are Wrong:

• A–D: Each segment is grammatical and natural; changing them would alter acceptable English without necessity.

Common Pitfalls:
Searching for nonexistent errors and changing idiomatic wording; confusing “certain” with certainty (here it means “some particular”).

Final Answer:
All correct