lsat test8 -sec2 -19\20

Questions 19-20

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-18 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 rain 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.

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.

(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18.   

(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen.

(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18.

(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains.

19答案B,20答案A，题意不大明白，请nn解释～～

Translation: O18重于正常的O的同位素。在积雨云中，含O18的水分子少于含正常O的水分子（假设为15%：85%）。但是在降雨过程中，在所有O18中，有更高比例的变成了雨水（假设为80%），而在普通O中变成雨水的比例较少（假设为55%）。后来，科学家全程跟踪测量了那些形成与大西洋上空并在Amazon上空变成降雨的积雨云。他们惊异的发现，在这些云里，O18的含量是不变的。