2. 外语
  3. There is now no doubt that the world is getting warmer. Data from both the Southern and Northern Hemispheres(地球的半球)shows a(n)【C1

There is now no doubt that the world is getting warmer. Data from both the Southern and Northern Hemispheres(地球的半球)shows a(n)【C1

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问题     There is now no doubt that the world is getting warmer. Data from both the Southern and Northern Hemispheres(地球的半球)shows a(n)【C1】______trend in average temperatures over the【C2】______hundred years. At the moment most scientists are not prepared to stick their necks out and【C3】______a cause to this trend. It is possible that it represents a natural climatic change caused by a【C4】______increase in the sun’s radiation output. But these same scientists are【C5】______aware that recent temperature changes are at the limit of known natural【C6】______: it seems increasingly likely that the cause is a(n)【C7】______greenhouse effect.
    The basic theory of the greenhouse effect is quite simple. The earth’s atmosphere consists【C8】______of oxygen and nitrogen,【C9】______there are small concentrations of various "greenhouse" gases-【C10】______carbon dioxide, water vapor, and methane(沼气)-- which play a very important role in【C11】______the planet’s "heat balance". As their name suggests these gases have a similar effect on glass in a greenhouse: They let heat from the sun【C12】______, and keep it in.
    The atmosphere is more or less transparent to the【C13】______solar radiation which warms the earth(ground or oceans)on which it falls.【C14】______it has been heated, the earth is warmer than space and gives【C15】______energy in the form of in visible long-wave infrared radiation. This【C16】______for nighttime cooling. Greenhouse gases【C17】______some of this long-wave radiation, and reradiate it in the lower atmosphere.
    The gases【C18】______act rather like a blanket by preventing some infrared radiation from leaving the earth-atmosphere system.【C19】______certain limits, the more greenhouse gases present, the more infrared radiation will become【C20】______, and the higher the surface temperature of the earth.
【C18】
选项 A、however
B、indeed
C、though
D、therefore
答案D
解析 逻辑衔接题。从上文的描述可知此处是对前文的总结，因此答案选 D therefore。
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