问题描述：
John passed the exam,although he didn't study very much for it
The exam _______ difficult.
A.can't have been
B.must have been
C.couldn't be
D.might have been
选哪一个 为什么
The exam _______ difficult.
A.can't have been
B.must have been
C.couldn't be
D.might have been
选哪一个 为什么
问题解答：
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