单项选择题

He has not quite solved the problem yet, but he is（）it.

A. making of
B. working on
C. clearing up
D. trying out

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（）Mary had heard that sound was she really frightened.

A. Not when
B. Not as
C. Not until
D. Until
单项选择题
Your father is seriously iii and you should（）a doctor at once.

A. send out
B. call in
C. go into
D. leave for
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