理工类A级：阅读理解Water
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 at 100℃. Life as we know it would only be possible on the surface of a planet which had temperatures somewhere within this narrow range.
The earth’s supply of water probably remains fairly constant in quantity. The total quantity of water is not know very accurately， but it is about enough to cover the surface of 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-about 97 percent. The rest is fresh， but three-quarters of this is in the form of ice at the Poles and on mountains， and cannot be used by living systems until melted. Of the remaining fraction， which is somewhat less than 1 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 atomosphere.
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 oceans than over the land， but there is more rainfall over the land than over the oceans， and the balance is restored by the runoff from the land in the form of rivers.
41 Liquid water only exists
A in the center of the earth.
B on the surface of our planet.
C in a very narrow range of temperatures.
D in the costal areas of the earth.
42 The total quantity of water on Earth
A remains almost unchanged.
B has greatly increased in recent years.
C is decreasing constantly.
D is affected by global warming.
43 Most of the fresh water on Earth
A is stored underground.
B is in the form of ice at the Poles and on mountains.
C is found in rivers and lakes.
D comes from the rain
44 The word “fraction” in the second paragraph means
A a very small amount.
B a large area
C an important system.
D a major source of information.
45 There is more rainfull
A over the mountains than over the rivers.
B over the rivers than over the mountains.
C over the oceans than over the land.
D over the land than over the oceans.
参考答案：C A B A D