单项选择题

More thunderstorms ______in summer than any other time of the year.

A．happened
B．have happened
C．happen
D．will happen
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单项选择题
Children don''t ______understand what they are reciting, but gradually it will have an impact on their thinking.
A．necessarily
B．profitably
C．unnecessarily
D．unprofitably
单项选择题
It seems oil_____ from this pipe for some time. We''ll have to take the machine apart to put it right.
A．had leaked
B．is leaking
C．leaked
D．has been leaking
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