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Difficulty: MediumModal Auxiliaries and Expressing Modality

Given that all digital activity logs were completely erased from the secure server at midnight, which modal verb phrase correctly completes the sentence below to express a past logical deduction based on strong evidence?

'The intruder _____ possessed high-level administrative credentials to bypass the multi-factor firewall.'

  1. must haveAnswer
  2. B
    must to have
  3. C
    should can have
  4. D
    ought have

Answer

The correct choice is 'must have', which appropriately expresses a strong logical deduction regarding a past event.
The structure 'must have' correctly expresses a logical deduction about a completed past action when supported by strong evidence.

Step-by-Step Solution

1
Determine the required communicative function of the modal verb from the contextual clue.
The clause 'all digital activity logs were completely erased' presents factual evidence pointing toward a highly probable conclusion about past credentials.
In English grammar, expressing a past logical deduction requires the perfective modal aspect ('modal + have + past participle').
2
Evaluate the structural correctness of the available modal options.
The auxiliary 'must' expresses certainty/deduction and takes a bare infinitive ('have'). The form 'must to have' is ungrammatical because primary modals do not accept 'to', 'should can have' is an invalid double modal, and 'ought have' lacks the required preposition 'to'.
Pure modal auxiliaries in English are followed directly by bare infinitives without 'to' and cannot be combined consecutively.

Key Concept

Modal Auxiliaries for Past Logical Deduction
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Topics

Modal Auxiliaries and Expressing Modality

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