单项选择题

They ______ us since five o'clock this morning until now.

A．are helping
B．help
C．have helped
D．have been helping
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单项选择题
If I could put back the clock, I'll give more thought to ______ for a career.
A．prepare
B．be prepared
C．preparing
D．have prepared
单项选择题
I ______ asleep in the corner, for I remember nothing of what happened during the night.
A．might fall
B．must fall
C．must have fallen
D．can have fallen
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