--It Jack who is at the gate of the library. --It him. I saw him off at the rail
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--It ____ Jack who is at the gate of the library.
--It ____ him. I saw him off at the railway station just an hour ago.
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
试题答案
B
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。