每单位体重，牛产生的二氧化碳最少。
Cattle produced the least carbon dioxide per unit of body mass.
天然气的二氧化碳产生量是每单位煤炭释放能量时二氧化碳产生量的一半。
Gas emits about half as much carbon dioxide as coal per unit of energy produced.
天然气的二氧化碳产生量是每单位煤炭释放能量时二氧化碳产生量的一半。
Gas emits about half as much carbon dioxide as coal per unit of energy produced.
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