单项选择题

When I try to understand ______ that prevents so many Americans from being as happy as one might expect, it seems to me that there are two causes.

A．why it does
B．what it does
C．what it is
D．why it is
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单项选择题
You cannot be ______ careful when you drive a car.
A．very
B．so
C．too
D．enough
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You don't have to be in such a hurry. I would rather you ______ on business first.
A．would go
B．will go
C．went
D．have gone
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