can't have been / mustn't have been

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Jenny ______ with him at that time, for I was having dinner with her in my home.
A. can’t have been
B. mustn’t have been
C. must be
D. may be
The answer is A.

I remember choice B has been discussed in this forum that it can be used to make speculation about past events. So what about B here?

Thank you in advance.

  • Matching Mole

    Senior Member
    England, English
    There is no speculation here. The speaker knows for a fact that Jenny was not with him, because she was with the speaker: she can't have been with him. "Mustn't" won't do in this case.
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