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 flights ______ because of the snowstorm, many passengers could do nothing but take the train.
A．had been canceled
B．have been canceled
C．were canceled
D．having been canceled
单项选择题
As teachers we should concern ourselves with what is said, not what we think______.
A．ought to be said
B．must say
C．have to be said
D．need to say
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