问题描述：
高中情态动词
-That must be a terrible mistake .-No.I don't think it __a mistake .A.must be B.mustn't be C.can be D can't be,
-That must be a terrible mistake .-No.I don't think it __a mistake .A.must be B.mustn't be C.can be D can't be,
问题解答：
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