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

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

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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 temperatures, 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 temperature of the sun. 　　　　     　             【B1】 ______
As we know, life would only be possible on the face　　　              【B2】 ______
of a planet had temperatures somewhere within this range.             【B3】 ______
The earth’s supply of water probably remains quite fairly             【B4】 ______
constant 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 about replaced by               【B5】 ______
new water rising away from the depths of the earth during             【B6】 ______
volcanic action. The total quantity of water is not known,
and it is about enough to cover the surface of the globe              【B7】 ______
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               【B8】 ______
in the form of ice at the Poles and on mountains,
and cannot be used by living systems when melted. Of the              【B9】 ______
remaining fraction, which is somewhat fewer than 1% of the            【B10】 ______
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 vapor in the atmosphere.
选项
答案在let与the之间加alone。
解析 let alone为固定表达，意思是“更不用说”。
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