问题描述：
There's someone outside.Who ___it be?
There's someone outside.Who ___it be?
It must be Mary.She promised to come here over at 8:00.
A:must B:can C:shall D:need
为什么?
There's someone outside.Who ___it be?
It must be Mary.She promised to come here over at 8:00.
A:must B:can C:shall D:need
为什么?
问题解答：
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