[求助]og-9-53

tania

现在在总结阅读，这是第9篇，对其中的53题的c选项有些疑问，看了以前的帖子，也还是不明白，想请大家帮忙解释一下：

53. It can be inferred from the passage that precipitation formed from evaporated ocean water has

(A) the same isotopic ratio as ocean water

(B) less oxygen 18 than does ocean water

(C) less oxygen 18 than has the ice contained in continental ice sheets

(D) a different isotopic composition than has precipitation formed from water on land

(E) more oxygen 16 than has precipitation formed from fresh water

c选项应回原文定位至黄色处：

To establish such a chronology it is necessary to

determine the relative amounts of land ice that existed

at various times in the Earth’s past. A recent discovery

(10) makes such a determination possible: relative land-ice

volume for a given period can be deduced from the ratio

of two oxygen isotopes, 16 and 18, found in ocean sedi-

ments. Almost all the oxygen in water is oxygen 16, but

a few molecules out of every thousand incorporate the

(15) heavier isotope 18. When an ice age begins, the conti-

nental ice sheets grow, steadily reducing the amount of

water evaporated from the ocean that will eventually

return to it. Because heavier isotopes tend to be left

behid when water evaporates from the ocean surfaces,

(20) the remaining ocean water becomes progressively

enriched in oxygen 18. The degree of enrichment can

be determined by analyzing ocean sediments of the

period, because these sediments are composed of calcium

carbonate shells of marine organisms, shells that were

(25) constructed with oxygen atoms drawn from the sur-

rounding ocean. The higher the ratio of oxygen 18 to

oxygen 16 in a sedimentary specimen, the more land ice

there was when the sediment was laid down.

问题1：我理解是growth of contiental ice sheets减少了水的蒸发数量，可是og 解释

C is incorrect because the passage suggests that the water evaporated from the ocean contributes to the growth of continental ice sheets, which should therefore have an isotopic composition similar to that of the precipitation formed from evaporated ocean water.

红色部分是怎么得出的呢？

问题2：that will eventually return to it 中that 和it分别指代谁？that指代water?it指代ocean?

可是这又与语法中定语从句优先修饰最近的名词冲突啊！困惑中。。。。

[此贴子已经被作者于2005-7-30 10:41:42编辑过]

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fuyun

黄线部分的意思是：冰期开始时，陆地上的冰层逐渐增长，相应的本应循环回海里的海水蒸发的水分就降低了。

og c的意思是：海里蒸发的水分变成了陆地增加的冰层，所以陆地冰层的同位素结构和海水蒸发的形成的雨雪是一样的。

that 指the amount of water evaporated from the ocean

it : the ocean