Water Resources onthe Earth

The second mostimportant constituent（构成成分）of the biosphere（生物圈）is liquid water. Thiscan 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 planetwhich had temperaturE. somewhere within this narrow range.

The earth's supplyof water probably remains fairly constant in quantity.The total quantity ofwater is not known very accurately,but it is about enough to cover the surfaceof

the globe to a depthof about two and three-quarter kilometers.Most of it is in the form of the saltwater of the oceans一about97 percent. The rest is fresh.But three-quarters

of this is in theform of ice at the poles and or mountains and cannot be used by living systemsuntil melted.Of the remaining fraction,which h somewhat less than one percent

of the whole.Thereis 10—20 times as much stored underground water as there is actually on thesurface.There is also a tiny,but extremely important fraction of the water

supply which ispresent as water vapour in the atmosphere.

Water vapour in theatmosphere is the channel through which the whole water circulation（循环） of the biosphere has to pass.Waterevaporated（蒸发）from the surface

of the oceans,fromlakes and rivers and from moist（潮湿的）earth is added to it. From it the water comes out again as rain orsnow,falling on either the sea or the land.

There is,as might beexpected, a more intensive evaporation per unit area over the sea and oceansthan over the land,but there is more rainfall over the land than over the

oceans and thebalance is restored by the run off from the land in the form of rivers.Most of the freshwater on Earth________.

A.is in the form ofice at the poles and on mountains

B.is storedunderground

C.is found in riversand lakes

D.comes from therain