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
I'd rather you ______ make any comment on the issue for the time being.
A．don't
B．wouldn't
C．didn't
D．shouldn't
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All things ______ , the planned trip will have to be called off.
A．considered
B．be considered
C．considering
D．having considered
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