单项选择题

Perhaps the days will come ______people will be able to breathe clean air in cities.

A．when
B．as
C．while
D．since
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单项选择题
The flyover at the crossing on the 6th ring road is now ______construction.
A．under
B．in
C．at
D．with
单项选择题
______when he saw his wife's face did Tom realize the true meaning of her remark.
A．Just
B．Never
C．Only
D．Usually
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