A Strong Greenhouse Gas

1 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.

2 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.

3 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.

4 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.

5 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.

6 That's why we say livestock gas is also a major factor of causing the global warming.

1. Paragraph 1

2. Paragraph 2

3. Paragraph 4

4. Paragraph 5

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

5. Methane is to the intensifying greenhouse effect.

6. Greenhouse gases are indispensable to mankind,but the problem mankind is faced with is .

7. Generally people heap criticism on for the planet's temperature rise.

8. Nothing has been mentioned in the passage about .

A one of the major contributors

B the ever-increasing atmospheric concentrations of greenhouse gases

C big and hefty cows

D livestock's normal digestion process

E how to cut down the cattle populations

F big industries and gas-guzzling vehicles

答案与题解：

1. E本段的第三句点出了 methane's heat-trapping power in the atmosphere is 21 times stronger than carbon dioxide.

2. F 本段要表达的中心意思是： livestock排出的甲烷也是温室效应的一个主要源头。

3. C本段表达的重点是用 but转折的段后两句。

4. D 本段从科学的角度分析反刍动物为什么会释放出甲烷。

5. A 选 A的依据在第一段的第二句和昀后一句。

6.B 选 B的依据在第三段的第二句、第三句和第四段的第一句。这三个句子先后介绍了下列观点：没有温室气体，生物不能生存；人类燃烧能源，加剧温室效应；大气甲烷含量增加一倍。

7. F 第四段第二句直接谈到这个问题。人们常常将不断加剧的温室效应归罪于大工业和汽车，而 cattle释放甲烷的因素被忽略了。

8. E 选项 E是本文没有提到的内容，也就是说，其他五个选项的内容文章部有涉及。