If the police had not intervened quickly, the suspected thief … by the mob.

Analyze the question The sentence is a conditional: "If the police had not intervened quickly, the suspected thief ........ by the mob." The if-clause uses the past perfect ("had not intervened"), so we need to identify which type of conditional this is and choose the matching main clause.

Explain the concept: the Third Conditional This conditional talks about an unreal or imagined situation in the past, something that did not happen, and its imagined result.

• Structure: If + past perfect, … would have + past participle
• Example: If she had studied, she would have passed. (She didn't study, so she didn't pass.)

Solve step by step

1. The if-clause is "had not intervened", which is past perfect, so it is a third conditional.
2. The main clause must therefore use would have + past participle.
3. The thief's action is passive (he is acted on by the mob), so we need would have been + past participle, giving "would have been killed".

Checking the options:

• A. would have been killed: correct third-conditional passive form.
• B. would be killed: this is the second-conditional pattern (If + past simple, would + verb), so it doesn't match.
• C. will have been killed: "will" does not pair with "had" in an if-clause, so it is wrong.
• D. will be killed: this is the first-conditional pattern (If + present, will + verb), so it is wrong.

Meaning check: The police did intervene, so the thief was not killed. "Would have been killed" expresses that imagined past outcome correctly.

Answer: A. would have been killed

Exam tip: Match the tense pattern of the if-clause:

• If + present → will
• If + past → would
• If + had + past participle → would have + past participle

Spot "had" right after "if" and you can eliminate the wrong options quickly.