Error Spotting – Choose the single segment (A–D) that contains a grammatical or usage error; select ‘‘All correct’’ only if the entire sentence is correct. Sentence: A) The faces of the B) twins were so C) identical that we could not D) differentiate between them

Introduction / Context:
This question examines precise verb choice in set expressions. When two people or things are very similar, the idiomatic verb is “distinguish,” not “differentiate,” in everyday English. “Differentiate between” is more common in technical contexts (calculus, classification), whereas “distinguish between” is the natural choice for telling two faces apart.

Given Data / Assumptions:

• The subject is the identical appearance of twins.
• The complement clause expresses inability to tell one from the other.
• We must pick the segment that needs revision to attain idiomatic correctness.

Concept / Approach:
Idiomatic collocations help readers process meaning quickly. “Distinguish between A and B” or simply “distinguish them” fits personal identification. “Differentiate” tends to describe functional or categorical differences rather than visual recognition of two individuals in casual discourse.

Step-by-Step Solution:
1) Parse the structure: so + adjective + that-clause indicates result.2) Evaluate verb choice for naturalness: replace “differentiate” with “distinguish”.3) Corrected sentence: “The faces of the twins were so identical that we could not distinguish between them.”4) Therefore, D contains the usage error.

Verification / Alternative check:
Try “tell them apart” as a paraphrase; it aligns with “distinguish,” reinforcing that D is the place to revise.

Why Other Options Are Wrong:

• A–C: Grammatically sound and semantically coherent within the result construction.

Common Pitfalls:
Believing that “differentiate” and “distinguish” are always interchangeable; in many real-world contexts they are not.

Final Answer:
differentiate between them