2. 外语
  3. Although there is considerable debate over how severe the greenhouse effect will be, one thing seems certain: carbon dioxide (二氧

Although there is considerable debate over how severe the greenhouse effect will be, one thing seems certain: carbon dioxide (二氧

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问题     Although there is considerable debate over how severe the greenhouse effect will be, one thing seems certain: carbon dioxide (二氧化碳) levels are on the rise. Measurements at the federal atmospheric observatory on Manna Loa volcano in Hawaii show that the concentration of the gas in the atmosphere has risen steadily from 315 parts per million in 1958 to 340 parts per million today.
    The event offender, scientists conclude, is the burning of coal, oil, synthetic fuels, and natural gas. These carbon-rich fossil (从地下采掘的) energy sources release an estimated five and a half billion tons of carbon into the atmosphere each year as colorless, odorless (无味的) CO2 gas.
    The increased carbon dioxide is probably not a threat to health, since normal indoor levels of the gas can run 1000 parts per million or higher without apparent harm. But it could deeply affect the way the earth is heated by the sun. The sun’s energy strikes the earth principally in the form of visible light. As the earth heats up, it radiates this energy back into space, but at the much longer wavelengths of infra- red (红外线的) light, or heat. Carbon dioxide lets the visible light pass through, but absorbs energy at infra-red wavelengths. Thus, the more carbon dioxide in the atmosphere, the more the earth’s heat is blocked from escaping --just as a blanket holds in the heat of a sleeper’s body.
    Some scientists think the greenhouse effect already has begun: average global temperatures have risen about one degree Fahrenheit in the past 90 years. Others argue that the rise could be due to the irregular change of natural temperature.
Carbon dioxide is now a threat to ______.
选项 A、people staying indoors
B、people in poor health
C、the earth’s vegetation
D、the earth’s temperature
答案D
解析 此题问二氧化碳主要对什么产生影响。第三段最后一句 “…the more the earth’s heat is blocked from escaping”，因而D是答案。其他三项均不符合题意。
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