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 意为“更不⽤说”， 表⽰⼀种让步关系。2.face-surface 根据上下⽂理解，应该指地球的表⾯。
3.planet^had 加⼊which或that。这样使得planet后⾯的句⼦成为⼀个定语从句。4.去掉fairly或quite 意思重复。5.out改成outer 这⾥指外层空间。6.去掉away rise后⾯不接away。
7.and改成although 短⽂中这⾥是让步关系，⽽不是并列关系。
8.quarter改成quarters 英语表达中，当分⼦是⼆以上的数时，分母应为复数。
9.when改成until或者unless 根据上下⽂，这句话的意思应为:除⾮（冰雪)融化，否则（地球上的）⽣命系统就⽆法利⽤⽔。所以when在这⾥的意思有误。
10.fewer改成less ⽂中的remaining fraction 仍指⽔，因此不能⽤fewer，要⽤less。
