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
______ cared about our sufferings like you.
A) Never before anyone has
B) Has anyone never before
C) Before has anyone never
D) Never before has anyone
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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