问题: 一、单项选择
1、Both his parents look sad.Maybe they _______what's happened to him.
A.knew
B.have known
C.must know
D.will know
解答:
Both his parents look sad.Maybe they ___B____what's happened to him.
A.knew
B.have known
C.must know
D.will know
表示对过去某事的肯定猜测:must have done.
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