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  3. The Excrement Pollutes Nearly 40 years after the first Earth Day, this is irony: The United States has reduced the manmade p

The Excrement Pollutes Nearly 40 years after the first Earth Day, this is irony: The United States has reduced the manmade p

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问题                     The Excrement Pollutes
    Nearly 40 years after the first Earth Day, this is irony: The United States has reduced the manmade pollutants that left its waterways dead, discolored and occasionally flammable . But now, it has managed to smother the same waters with the most natural stuff in the world. Animal manure, a byproduct as old as agriculture, has become a modern pollution problem, scientists and environmentalists say. The country simply has more dung than it can handle: Crowded together at a new breed of big farms, livestock produce three times as much waste as people, more than can be recycled as fertilizer for nearby fields.
    That excess manure gives off air pollutants, and it is the country’s fastest-growing large source of methane, a greenhouse gas. And it washes down with the rain, helping to cause the 230 oxygen-deprived "dead zones" that have boosted along the U.S. coast. In the Chesapeake Bay, about one-fourth of the pollution that leads to dead zones can be traced to the back ends of cows, pigs, chickens and turkeys. The reasons for manure’s rise as a pollutant have to do, environmentalists say, with a shift in agriculture and a soft spot in the law.
    In recent decades, livestock raising has shifted to a smaller number of large farms. At these places, with thousands of hogs or hundreds of thousands of chickens, the old self-contained cycle of farming-manure feeds the crops, then the crops feed the animals— is overwhelmed by the large amount of waste. The result in farming heavy places has been too much manure and too little to do with it. In the air, that extra manure can dry into dust, forming a "brown fog". It can emit substances that contribute to climate change. And it can give off a smell like a punch to the stomach. "You have to cover your face just to go from the house to the car," said Lynn Henning, 52, a farmer in rural Clayton, Mich. , who said she became an environmental activist after smoke from huge new dairies gave her family headaches and burning sinuses.
    In the water, the chemicals in manure don’t poison life, like pesticides or spilled oil. Instead, they create too much life, and the wrong kinds. The chemicals in manure serve as fertilizer for unnatural algae blooms. They drain away oxygen as they decompose. resulting increasing number of suffocating dead zones.
The overwhelming amount of manure has almost broken the old self-contained______.
选项
答案cycle of farming
解析 根据题干关键词the old self-contained定位到第三段。第二句：At theseplaces，with thousands of hogs or hundreds of thousands of chickens，the old self containedcycle of farming．．．is overwhelmed by the large amount of waste．可知，传统的自给自足的农业模式被大量的动物粪便破坏。
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