单项选择题

It's high time we ______ something to stop air pollution.

A．are doing
B．did
C．have to do
D．must do
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单项选择题
It is useful to be able to predict the extent ______ which a price change will affect supply and demand.
A．with
B．from
C．for
D．to
单项选择题
It was not until he arrived at the station ______ he realized he had forgotten his ticket.
A．when
B．that
C．for
D．in
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