单项选择题

What is the most dangerous thing about nitrous oxides

A．Stagnant air.
B．Damage to concrete.
C．Unfounded fears.
D．Potential carcinogenic effects.
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单项选择题
According to the woman, what causes most of the damage to the air
A．Industrial sources.
B．Automobiles.
C．Nitrogen sources.
D．Natural activities.
单项选择题
What steps could be taken to improve the air
A．Reduce the amount of nitrogen.
B．Increase the release of oxygen.
C．Reduce nitrous oxide emissions.
D．Stop industrial activities.
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