[单项选择]第一篇 　　 Water 　　The second most important constituent (构成成份) 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 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 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 fractional which is somewhat less than one per cent of the whole, there is 10—20 times as much stored underground wat
A. a very small amount.
B. a large area.
C. an important system.
D. a major source of information