单项选择题
I can't ______ what I am doing because it is so noisy here.
A．concentrate
B．concentrate in
C．concentrate on
D．concentrate about
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单项选择题
The atmosphere is as much a part of the earth as ______ its soils and the water of its lakes, rivers and oceans.
A．do
B．has
C．is
D．are
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单项选择题
It is high time we ______ this practice.
A．stop
B．stopped
C．have stopped
D．had stopped
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