Choose the modal verb phrase that correctly expresses logical impossibility in the past:

Given that the laboratory access logs show Dr. Lee was in Tokyo on Monday, she ________ the unauthorized biological synthesis in Kuala Lumpur that morning.

Answer and explanation

Correct answer: C. could not have executed

Answer: could not have executed. Why not the others: 'must not execute' expresses present prohibition, not past deduction; 'might not have executed' expresses possibility or doubt rather than deductive impossibility; 'should not have executed' expresses moral censure or breach of protocol for an act that was actually committed.
  1. Step 1: Analyze the epistemic requirement: the premise provides a conclusive alibi ('she was in Tokyo'), making her physical presence in Kuala Lumpur logically impossible.
  2. Step 2: Negative past logical deduction (impossibility) is expressed by 'cannot have' or 'could not have + past participle'.

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