Oxygen-18 is a heavier-than-normal isotope of oxygen. In a rain cloud

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.

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. - CORRECT. Already stated in passage though not explicitly but again this is how it is.
(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18. - WRONG. Whether greater or same or less, we are not sure if it's true. All three possibilities are equally likely or may be(all) not.
(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen. - WRONG. "Rain cloud" not "rainfall" is what we are looking for. Latter part is still considerable.
(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18. - WRONG. Not sure about same percentage of molecules.
(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains. - WRONG. Problem is with retain part which we are not sure about. Thus, we can't infer.

Used POE to get the right answer but sadly in 4+ minutes. Took time mostly for the reason that options are confusing except the right one. So, a tough question the answer of which seems to easy to believe.
Take note of what is asked in question stem. It's about rain cloud and not rainfall. So, anything about rainfall is out of scope or irrelevant, however, in most likely terms and not in absolute terms.

Answer A.

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