U-Substitution 换元法
u substitution
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以上为机器翻译结果，长、整句建议使用 人工翻译 。
Finally, the substitution of natural gas for coal to generate electricity has reduced carbon dioxide emissions in the U.S. to their lowest level in 20 years.
FORBES: The Choice Of 2012: Obama Austerity vs. Romney Growth
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