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
The medicine is on sale everywhere. You can get it at ______ chemist's.
A．each
B．some
C．certain
D．any
单项选择题
______ it left to me to decide, I would never hesitate to choose the former.
A．If
B．Were
C．Had
D．Should
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