The second most important constituent of the biosphere is
liquid water. This can only exist in a very narrow range of temperatures,
since water freezes at 0°C and boils at 100°C. This is
only a tiny range compared with the low temperatures of some
other planets and the hot interior of the earth, let the temperature __1__
of the sun.
As we know, life would only be possible on the face of a __2__
planet had temperatures somewhere within this range. __3__
The earth’s supply of water probably remains quite fairly constant __4__
in quantity. A certain number of hydrogen atoms, which
are one of the main constituents of water, are lost by escaping
from the atmosphere to out space, but they are probably just __5__
about to be replaced by new water rising away from the depths of the __6__
earth during volcanic action. The total quantity of water is not
known, and it is about enough to cover the surface of the globe __7__
to a depth of about two and three-quarter kms. Most of it—97%
is in the form of the salt waters of the oceans. The rest is
fresh, but three quarter of this is in the form of ice at the Poles __8__
and on mountains, and cannot be used by living systems when __9__
melted. Of the remaining fraction, which is somewhat fewer __10__
than 1%of the whole, there is 10-20 times as much stored as underground
water as is actually on the surface. There is also a minor,
but extremely important, fraction of the water supply
which is present as water vapour in the atmosphere.

答案：
1.Let ^the 加入alone. Let alone 意為“更不用說”， 表示一種讓步關(guān)系。
2.face-surface 根據(jù)上下文理解，應(yīng)該指地球的表面。
3.planet^had 加入which或that。這樣使得planet后面的句子成為一個(gè)定語從句。
4.去掉fairly或quite 意思重復(fù)。
5.out改成outer 這里指外層空間。
6.去掉away rise后面不接away。
7.and改成although 短文中這里是讓步關(guān)系，而不是并列關(guān)系。
8.quarter改成quarters 英語表達(dá)中，當(dāng)分子是二以上的數(shù)時(shí)，分母應(yīng)為復(fù)數(shù)。
9.when改成until或者unless 根據(jù)上下文，這句話的意思應(yīng)為:除非（冰雪)融化，否則（地球上的）生命系統(tǒng)就無法利用水。所以when在這里的意思有誤。
10.fewer改成less 文中的remaining fraction 仍指水，因此不能用fewer，要用less。