Answer and explanation

Correct answer: A. to have left

  1. Step 1: 'Last night' is earlier than 'is believed', so the perfect infinitive is needed.
Answer
to have left.
Why not the others: "to be left" is wrong because the passive does not fit; the suspect is the doer. "to leave" is wrong because the simple infinitive refers to a present or future time, not to 'last night'. "leaving" is wrong because the -ing form is not used after the passive 'believed'.

Practise more Passives, gerunds, infinitives and modals.

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