Why has China overtaken the United States as the top energy consumer in the
world?

Voracious energy demand also helps explain why China—which gets most of its electricity from coal, the most polluting of fossil fuels—passed the U.S. in 2007 as the world's largest emitter of carbon-dioxide emissions and other greenhouse gases.

Explanation:

i'm sure the answer you are looking for would be

a. an ice age

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