-I think the man over there must be Bob.-It ____ be him.He has gone to Australia.

题目

-I think the man over there must be Bob.-It ____ be him.He has gone to Australia.
空格处为什么要用can't,不能用mustn't

答案

这是否定的推测 只能用can't
mustn't表示“必须不”表示禁止类似的意思,不可用作推测