单项选择题

A．It exists close to the surface of the Earth.
B．It is a component of smog and a harmful substance.
C．It is a kind of yellow gas.
D．It is formed through a complex reaction.
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The country that brings about the largest amount of global warming pollution is______.
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______are used by several automakers to produce hybrid sedans, SUVs and trucks, which help cut global warming pollution.
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