单项选择题

A．when
B．one
C．once
D．what
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单项选择题
What gas does not pollute the air
A．Oxygen.
B．Hydrocarbons.
C．Carbon monoxide.
D．Nitrogen oxides.
单项选择题
How many workers were fired
A．About 670.
B．About 1000.
C．About 10.
D．About 80.
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