单项选择题

What gas does not pollute the air

A．Oxygen.
B．Hydrocarbons.
C．Carbon monoxide.
D．Nitrogen oxides.
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单项选择题
What can an exhaust system do if it is efficient
A．It can prevent the waste gas from escaping.
B．It can let the waste gas and compound escape quickly out of it.
C．It can prevent the engine from producing waste gas.
D．It can store as much waste gas as possibl
单项选择题
What is the passage mainly about
A．The exhaust system of a car.
B．Waste gas.
C．The car engine.
D．Air pollution.
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