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
The United States and Canada have agreed ______ in regard to the Great Lakes.
A．to eliminate swimming entirely
B．to limit boating and
C．to limit phosphates and detergents
D．to kill all algae
单项选择题
______ has been the major problem in the Great Lakes in recent years.
A．Too many algae
B．Excessive recreational use
C．Lack of adequate rainfall
D．Too much evaporation
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