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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单项选择题
（）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
You have worked too hard and you'd better take a holiday northward or（）

A．however
B．whatever
C．wherever
D．whichever
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