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. Ann: I told her to go upstairs and wait there. Bob: She ……………………………….. - she’s still here.

  1. a) wasn’t able understand b) mustn’t understood c) can’t have understood d) didn’t understood

why the correct answer is the c?

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a) wasn’t able to understand - This is not a grammatical expression. "to" is necessary. Even with the infinitive, it is not a correct answer.

b) mustn’t understood This is not a grammatical expression. The base form of the verb is used after a modal.

c) can’t have understood - This is the modal perfect form (modal auxiliary + perfect infinitive) and is exactly correct.

d) didn’t understood - This is not a grammatical expression. The base form of the verb is used after the helping verb "do".


Here is a site that describes modals + perfect infinitive: https://7esl.com/perfect-infinitive-with-modals/

Examples:

Modal + have + past participle (active voice) or Modal + have + been + past participle (passive voice)

The car shouldn't have broken down. (It did break down.) I just had a mechanic check it over last week.
She must have been at the party last night. Jeremy had a message from her best friend who saw her there.
The computer must have been rebooted. I had to log back in to read my email. (passive)
She couldn't have taken her cell phone with her. I had borrowed it from her yesterday and still had it in my pocket.
My cell phone can't have been stolen. I am holding it in my hand. (passive)

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miky98

. Ann: I told her to go upstairs and wait there. Bob: She ……………………………….. - she’s still here.

  1. a) wasn’t able understand b) mustn’t understood c) can’t have understood d) didn’t understood

Why is the correct answer c? is the c?

If she understood that she was supposed to go upstairs, she would be upstairs. But she's not upstairs. She's still here.

So it is not possible that she has understood. It can't be true that she has understood. > She can't have understood.

CJ

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Comments  

I don't know either! There is no correct choice in my opinion! It sounds chaos.


I'm curious to read the teachers' correction too.

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