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

答案與題解：

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

2. F 本段要表達的中心意思是： livestock排出的甲烷也是溫室效應的一個主要源頭。

3. C本段表達的重點是用 but轉折的段后兩句。

4. D 本段從科學的角度分析反芻動物為什么會釋放出甲烷。

5. A 選 A的依據在第一段的第二句和昀后一句。

6.B 選 B的依據在第三段的第二句、第三句和第四段的第一句。這三個句子先后介紹了下列觀點：沒有溫室氣體，生物不能生存；人類燃燒能源，加劇溫室效應；大氣甲烷含量增加一倍。

7. F 第四段第二句直接談到這個問題。人們常常將不斷加劇的溫室效應歸罪于大工業(yè)和汽車，而 cattle釋放甲烷的因素被忽略了。

8. E 選項 E是本文沒有提到的內容，也就是說，其他五個選項的內容文章部有涉及。