Oxygen.18 is a heavier-than-normal isotope of oxygen. In a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen. But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth. Consequently, scientists were surprised when measurements along the entire route of rain clouds' passage from above the Atlantic Ocean, the site of their original formation, across the Amazon forests, where it rains almost daily, showed that the oxygen-18 content of each of the clouds remained fairly constant.
20. Which one of the following inferences about an individual rain cloud is supported by the passage?
(A) Once it is formed Over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18. 原文复述：In a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen.
(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18. 原文不可推出。
(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen. 原文复述：But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth.
(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18. 原文不可推出。
(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains. 原文没讲，但是可以从一下2句话中推出：（1）In a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen. （2）But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth.因为在rain cloud中，O-18比常态少，而在rainfall中，O-18又比常态多，因此the cloud must surrender more of its oxygen-l8 than it retains。
以下是引用dorbear在2003-9-22 6:33:00的发言： 搂主在推断中不自觉的加入了一个主观条件，即rain fall 中的 O-18 就是rain cloud中的O-18。
2偏贴子，特别是这句话，看得出dorbear很有些功底。
即使这个假设成立。（我认为该假设成立。）E还是一个错误答案。为什么？因为E的准确中文是：在降雨的过程中，透支了O-18.During the rainfall, the cloud surrender more of its O-18 than it retins.原来我将E理解为：在降雨的过程中，放弃了大部分O-18.理解有误。
A还是原文推论。文章在谈O-18与普通氧分子的关系是没有说 in the original formation的情况。因此这里还有一个假设，就是after the formation of the rain could but before the rainfall happens, the relationship between the O-18 and normal oxygen will not change significantly.
(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen. 原文复述：But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth. 原文：雨水中含有O18的水分子的比例高于含正常氧的水分子。
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发表于 2003-9-19 01:33:00
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