【参考答案】1—5、ABDBC

　　阅读下列四篇短文，从每小题后所给的A,B,C或D四个选项中，选出最佳选项。

　　Rivers may be a significant source of the greenhouse gas nitrous oxide (一氧化二氮), scientists now find.

　　Their calculation suggests that across the globe the waterways contribute three times the amount of nitrous oxide to the atmosphere as had been estimated by the International Panel on Climate Change (IPCC), the United Nations scientific body charged with reviewing climate change research. They found that the amount of nitrous oxide produced in streams is related to human activities that release nitrogen (氮) into the environment, such as fertilizer use and sewage discharges.

　　“Human activities, including fossil fuel combustion and intensive agriculture, have increased the availability of nitrogen in the environment,” said Jake Beaulieu of the University of Notre Dame and the U.S. Environmental Protection Agency in Cincinnati, Ohio, and lead author of the paper published this week in the journal Proceedings of the National Academy of Sciences.

　　“Much of this nitrogen is transported into river and stream networks,” Beaulieu said. There, microbes (微生物) convert the nitrogen into nitrous oxide (also called laughing gas) and an inert gas called dinitrogen (二氮).

　　The finding is important, the researchers say, because nitrous oxide is a powerful greenhouse gas that contributes to climate change and destruction of the stratosphere’s ozone layer, which protects us from the sun’s harmful ultraviolet (紫外线) radiation. Compared with carbon dioxide, nitrous oxide is 300-fold more powerful in terms of its warming potential, though carbon dioxide is a far more common greenhouse gas. Scientists estimate nitrous oxide accounts for about 6 percent of human-induced climate change.

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