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.
19. Which one of the following statements, if true, best helps to resolve the conflict between scientists' expectations, based on the known behavior of oxygen-18, and the result of their measurements of the rain clouds' oxygen-IS content? (A) Rain clouds above tropical forests are poorer in oxygen-18 than rain clouds above unforested regions. (B) Like the oceans, tropical rain forests can create or replenish rain clouds in the atmosphere above them. (C) The amount of rainfall over the Amazon rain forests is exactly the same as the amount of rain originally collected in the clouds formed above the Atlantic Ocean. (D) The amount of rain recycled back into the atmosphere from the leaves of forest vegetation is exactly the same as the amount of ram in river runoffs that is not recycled into the atmosphere. (E) Oxygen-18 is not a good indicator of the effect of tropical rain forests on the atmosphere above them.
以前的回贴http://forum.chasedream.com/dispbbs.asp?boardID=24&replyID=20636&ID=3700&skin=1 —————————————————————————————————————— 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. O18 (含Oxygen.18的水分子) 的比例很低，与O (含普通Oxygen.的水分子)， 句题多多少不知道设为X
But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth. 降雨时，
所有O18 降下的比例 （proportion of all water molecules containing oxygen-18）
高于
所有 O 降下的比例 （proportion of all water molecules containing ordinary oxygen）
所以 如果下雨 O18在rain cloud中的比例 因该比以前 X 更低， 所以引出下面的科学家的困惑
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.
所有的云中的O18的比例不变？？ 所以一种可能是在移动的过程中没下雨，这正好是C, 答案确是B
(B) Like the oceans, tropical rain forests can create or replenish rain clouds in the atmosphere above them.
(C) The amount of rainfall over the Amazon rain forests is exactly the same as the amount of rain originally collected in the clouds formed above the Atlantic Ocean.
B 只能所提供了可能性，而且这种可能还需要在补从的雨云中， O18的比例要高于 the clouds formed above the Atlantic Ocean.
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发表于 2003-9-12 02:43:00
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