LSAT-8-2-19/20

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-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.

(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.

The key is B and A for 19 and 20 separately. Why? I feel difficult to understand the topic.

理解了原文就容易：在云中，含18氧的水分子比含正常氧的水分子少。但在下雨时，较大比例的含18氧的水分子降到地面。（这样就造成云中含18氧在下雨后应该减少）。但科学家奇怪的发现云从在Atlantic Ocea上空形成，到老下雨的Amazon forests。整个过程18氧含量并没变化。

19，20题就简单啦。19题B补充18氧。20题A是原文第一句话