I'm trying to understand why answer is supposed to be B here:
She's got a terrible mark in the exam so she ....very hard at all.
A. mustn't have worked
B. can't have worked
C. didn't work
My answer is A.
I can't find any rules how to explain it.
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possible duplicate of Can "mustn't" be used for conclusions?. But for your purposes you might be better off considering English Language Learners, where He mustn't / couldn't have been hungry is well-covered. – FumbleFingers Jan 30 '14 at 21:18
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This question was asked, see: What is the difference between "can't" and "mustn't" in the expressing of prohibition?
Mustn't implies She "should not" have.. Can't implies She "could not" have..
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2If the question already has an answer, you should flag the question as a duplicate rather than give a link to the question in an answer (if flagging requires more reputation than you currently have, just stick around and find some questions you can give actual answers to, or ask questions of your own, and you'll quickly gain enough reputation to be able to flag questions). – Janus Bahs Jacquet Jan 31 '14 at 00:43
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What @Janus said. For which reason I wouldn't normally upvote the answer, but I have on this occasion simply to balance out what I see as an unjustified downvote (it may be superfluous, and not a particularly good answer, but it's not incorrect or of low enough quality to be rejected out of hand). – FumbleFingers Jan 31 '14 at 12:49