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
All the key words in the article are printed in______ type so as to attract readers'' attention.
A．dark
B．dense
C．black
D．bold
单项选择题
We are all for your proposal that the discussion______.
A．be put off
B．was put off
C．should put off
D．is to put off
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