Question ID:PT18 S2 Q20 Passage: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. Stem:Which one of the following inferences about an individual rain cloud is supported by the passage? Correct Answer Choice:AChoice A:Once it is formed over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18. Choice B:Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18. Choice C:The cloud’s rainfall contains more oxygen-18 than ordinary oxygen. Choice D:During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18. Choice E:During a rainfall, the cloud must surrender more of its oxygen-18 than it retains.