--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 beB.must be; can't beC.can be; must have beenD.must be; can't have been
网友回答
B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。