B. House-moving companies are busy because their houses are in danger.

　　C. To stop permafrost melting is a very difficult problem.

　　D. Most of Alaska’s land surface has permafrost.

　　【参考答案】21.BADB

　　阅读理解

　　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).

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