题目内容：
回答题：A Strong Greenhouse Gas Methane is a colorless, odorless gas; it is also a potent greenhouse gas, and once released into the atmosphere1, it absorbs heat radiating from Earth’s surface. That’s why methane is a major contributor to the planet’s increasing temperature rise-or global warming. Molecule for molecule, methane’s heat-trapping power in the atmosphere is 21 times stronger than carbon dioxide2, the most abundant greenhouse gas.
With 13 billion cows belching almost constantly around the world (100 million in the U. S. alone), it’s no surprise that methane released by livestock is one of the chief global sources of the gas. Other prime methane sources: petroleum, drilling, coal mining, solid-waste landfills and wet lands.
Greenhouse gases like methane and carbon dioxide make up only a small part of Earth’s atmosphere, which is 78 percent nitrogen and nearly 21 percent oxygen. And without greenhouse gases to trap the sun’s heat and warm the planet, life as we know it couldn’t exist3. But in the last 200 years, human activity that requires burning oil, natural gas, and coal for energy has magnified the greenhouse effect.
Atmospheric concentrations of methane have more than doubled in the last two centuries. Blame for this often focuses on big industries and gas-guzzling vehicles. But agriculture plays a major role, too. In the past 40 years alone, the global cattle population has doubled.
Cows munch mostly grasses and hay-yet they grow big and hefty. Why? Because of the rumen. The rumen holds 160 liters of food and billions of microbes. These microscopic bacteria and break down cellulose and fiber into digestible nutrients. A cow couldn’t live without its microbes. As the microbes digest cellulose, they release methane. The process occurs in all animals with a rumen (cows, sheep, and goats, for example), and it make them very gassy. It’s part of their normal digestion process. When they chew their cud, they regurgitate some food to rechew it, and all this gas comes out. The average cow expels 600 liters of methane a day.
That’s why we say livestock gas is also a major factor of causing the global warming.
Paragraph 1______ A．Life of Microscopic Bacteria in Livestock’s Rumen
B．Ways to Reduce Methane’s Heat-Trapping Power
C．Agriculture Also Contributes to Increased Concentrations of Methane in the Atmosphere
D．Why Livestock Releases Methane
E．Methane as a Strong Greenhouse Gas
F．Livestock as a Prime Factor of the Greenhouse Effect
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