一道问过的题

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-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.

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.

(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.

20题答案A，C为什么不对？之前的讨论没有讲到C