题目内容：

说明理由
1.— Someone is knocking at thedoor.Is it Ann?
— It _____ be her.She is giving a performance at thetheatre now.
A.may B.must C.can’t D.mustn’t

优质解答

选C,can't意思是不可能.must是肯定是的意思.这两个都是表示推测,must用在肯定句中,can't是must的否定.may是也许,mustn't是禁止.