Question

--It ____ Jack who is at the gate of the library.

--It ____ him. I saw him off at the railway station just an hour ago.

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A．can be; can't be

B．must be; can't be

C．can be; must have been

D．must be; can't have been

Answer 1

答案：
解析：

must用于表示肯定推断，意为“想必”“一定”，表示否定推断，不可能时却不用mustn't，而用can't。本题两处空白都是对现在情况的推断，故用不定式一般式。