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  3. 共用题干 第一篇Water Resources on the EarthThe second most important constituent（构成...

共用题干 第一篇Water Resources on the EarthThe second most important constituent（构成...

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问题 共用题干
第一篇

Water Resources on the Earth

The second most important constituent（构成成分）of the biosphere（生物圈）is liquid water. This can only exist in a very narrow range of temperatures,since water freezes at 0℃and boils a 100℃．Life as we know it would only be possible on the surface of a planet which had temperature: somewhere within this narrow range.
The earth's supply of water probably remains fairly constant in quantity.The total quantity of water is not known very accurately,but it is about enough to cover the surface of the globe to a depth of about two and three-quarter kilometers.Most of it is in the form of the salt water of the oceans一about 97 percent. The rest is fresh.But three-quarters of this is in the form of ice at the poles and or mountains and cannot be used by living systems until melted.Of the remaining fraction,which h somewhat less than one percent of the whole.There is 10—20 times as much stored underground water as there is actually on the surface.There is also a tiny,but extremely important fraction of the water supply which is present as water vapour in the atmosphere.
Water vapour in the atmosphere is the channel through which the whole water circulation（循环） of the biosphere has to pass.Water evaporated（蒸发）from the surface of the oceans,from lakes and rivers and from moist（潮湿的）earth is added to it. From it the water comes out again as rain or snow,falling on either the sea or the land. There is,as might be expected, a more intensive evaporation per unit area over the sea and oceans than over the land,but there is more rainfall over the land than over the oceans and the balance is restored by the run off from the land in the form of rivers.
选项 The word"fraction"in the second paragraph means________.
A:a large area
B:a very small amount
C:an important system
D:a major source of information
答案B
解析文章第一段的第二句讲到液态水“only exist in a very narrow range of temperatures"，虽然B（我们星球的表面）也有点道理，但是液态水对温度的要求很高，并非地球的所有表面都有液态水存在，所以答案D正确。
第二段第一句提到地球上水的供给“remains fairly constant in quantity"，数量上是恒定的，它就没有增加也不会减少，只不过会以不同的形态出现，所以选择B（保持不变）。
问题是“地球上绝大部分淡水以什么形式出现？”第二段中间提到淡水时，说到“three-quarters of this is in the form of ice at the poles and on mountains"，即3/4的淡水以冰的形式存在于两极和高山中，后面提到剩余部分的淡水存在于地下，但占比少于1%,所以选项A正确。
词义辫析题， fraction有“小部分”的意思，考虑到前面提到地球97％的水以海洋咸水的形式存在，剩下的3％则以冰的形式存在于两极和高山，余下的自然不多，所以选B。
第三段的最后一句说到“there is more rainfall over the land than over the oceans”，陆地的降雨比海洋要多，但没有提到河流和山脉的降雨比较，所以C正确。大气中的水蒸气是整个生物圈的水循环必经的途径。水从海洋的表面、湖泊、河流和潮湿的泥土中蒸发出来并补充到大气中。水以雨或雪的形式再次出现，降落在海洋或陆地上。可以预料到的是，海洋上空每单位面积的水蒸气比在陆地上空更为密集，但是，陆地比海洋有更多的降雨，水在陆地上以河流的形式流出以保持水资源的平衡。
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