问题补充：
Mr. Smith has promised that all of us will pass the exam, so it so difficult as you imagine.
A. can’t have been
B. can’t be
C. mustn’t have been
D. mustn’t be
答案：
A. can’t have been
(答案→)B. can’t be
C. mustn’t have been
D. mustn’t be
解析：考察情态动词用法。AC是对过去情况的说明。D不能表示推测。Can’t be不可能；结合句意可知B正确。句意：史密斯先生答应所有人都能通过考试。所以试卷不可能像你想象的那么难。
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