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.

1. A.
   can be; can't be
2. B.
   must be; can't be
3. C.
   can be; must have been
4. D.
   must be; can't have been

Answer 1

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。