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问题: 情态动词

-I telphoned you yesterday afternoon,but no one answerd.
-Really?My mother was in then,but she _in bed.
A.must be sleeping
B.must have been sleeping
C.must have slept
D.should have slpet
请说明理由

解答:

选A,由题意可知,文中的那个人打给他朋友电话却没有人接电话,而当时他母亲又在家里,所以可以断定他母亲一定在睡觉。故选A。