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Difficulty: MediumModal Auxiliaries and Expressing Modality

The chief technician noted that the automated cooling units activated immediately after the temperature spike, which indicates that the emergency backup system ________ running properly at the time. Which of the following modal verb expressions correctly completes the sentence to express a strong logical deduction about a past action?

  1. A
    might can have been
  2. must have beenAnswer
  3. C
    must to have been
  4. D
    should could have been

Answer

must have been
The expression 'must have been' is correct because 'must' + perfect infinitive ('have' + past participle) expresses a strong, logical deduction about a completed past action based on given evidence.

Step-by-Step Solution

1
Analyze the communicative function required by the context.
The phrase 'which indicates that' signals that a logical conclusion/deduction is being drawn based on past empirical evidence.
Modal auxiliaries express different degrees of modality such as obligation, permission, capability, or logical deduction.
2
Determine the correct modal verb structure for past logical deduction.
Past logical deduction requires the modal verb 'must' followed by the perfect infinitive ('have' + past participle).
'Must have been' conveys high certainty about a past event.
3
Evaluate the grammatical validity of each distractor.
Options containing double modals ('might can', 'should could') or modal + to-infinitive ('must to') violate English auxiliary syntax rules.
English modal auxiliaries cannot be combined in sequence and must be followed directly by a bare infinitive.

Key Concept

Expressing past logical deduction using modal auxiliaries
Estimated Time:1m 0s
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Topics

Modal Auxiliaries and Expressing Modality

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