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.
