单项选择题

Not until I began to work ______ how difficult it was.

[A] I knew
B. I did know
C. did I know
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单项选择题
It ______ heavily so there was deep snow on the road.
[A] snows
B. snowed
C. had snowed
单项选择题
She came into the room while they ______ television.
[A]have watched
B.have been watching
C.were watching
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