单项选择题

When I try to understand ______ that prevents so many Americans from being as happy as one might expected, 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
They are considering ______ the house before the prices go down.
A. selling
B. of selling
C. to sell
D. over selling
单项选择题
Making energy use completely harmless to the environment ______ very difficult and usually economically expensive.
A. is



B. are
C. have been


D. shall be
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