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  3. WATER The second most important constituent of the biosphere is liquid water. This can only exist in a very narrow range of

WATER The second most important constituent of the biosphere is liquid water. This can only exist in a very narrow range of

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问题                             WATER
    The second most important constituent of the biosphere is
liquid water. This can only exist in a very narrow range of tem-
peratures, since water freezes at 0℃ and boils at 100℃. This is
only a tiny range compared with the low temperatures of some
other planets and the hot interior of the earth, let the tempera-       【M1】______
ture of the sun.
As we know, life would only be possible on the face of a                【M2】______
planet had temperatures somewhere within this range.                    【M3】______
The earth’s supply of water probably remains quite fairly con-          【M4】______
stant 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            【M5】______
about replaced by new water rising away from the depths of the          【M6】______
earth during volcanic action. The total quantity of water is not
known, and it is about enough to cover the surface of the globe         【M7】______
to a depth of about two and three-quarter ms. 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     【M8】______
and on mountains, and cannot be used by living systems when             【M9】______
melted. Of the remaining fraction, which is somewhat fewer              【M10】______
than 1% of the whole, there is 10  20 times as much stored as un-
derground water as is actually on the surface. There is also a mi-
nor, but extremely important, fraction of the water supply
which is present as water vapour in the atmosphere.
【M5】
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