Self-study
Oxygen-18 is a heavier-than-normal isotope of oxygen. ██ █ ████ ██████ █████ █████████ ██████████ █████████ ███ █████ ████ █████ █████████ ██████████ ██████ ███████ ███ ██ █████████ █ ██████ ██████████ ██ ███ █████ █████████ ██████████ █████████ ████ ██ ███ █████ █████████ ██████████ ████████ ██████ ████████ ██ ██████ █████████████ ██████████ ████ █████████ ████ ████████████ █████ ███ ██████ █████ ██ ████ █████████ ███████ ████ █████ ███ ████████ ██████ ███ ████ ██ █████ ████████ ██████████ ██████ ███ ██████ ████████ █████ ██ █████ ██████ ██████ ██████ ████ ███ █████████ ███████ ██ ████ ██ ███ ██████ ████████ ██████ █████████
Structure: Surprising Phenomenon
The stimulus tells us that water molecules containing oxygen-18, a heavier isotope of oxygen, are rarer in rainclouds than water molecules containing regular oxygen, but that when rain falls, a higher proportion of the water molecules containing oxygen-18 than of the regular water molecules falls to earth. However, when scientists tracked the whole route of rain clouds' passage from the Atlantic across the Amazon forests, they found that the oxygen-18 content in the rain clouds stayed relatively constant, despite it raining almost daily.
Analysis: Ways to Reconcile
The reason this scenario is surprising is that, if a higher proportion of water molecules containing oxygen-18 than of regular water molecules falls from rain clouds when it rains, then, in a situation like the one the scientists are observing, where it rains almost daily, you would expect the oxygen-18 content of the rain clouds to go down. In short, we want to explain why the oxygen-18 content of the rain clouds on this route was apparently not depleted. A good way to explain this would be if we knew about some other natural process by which rain clouds took up more oxygen-18 and so "replenished" the oxygen-18 level.
19.
Which one of the following ███████████ ██ █████ ████ █████ ██ ███████ ███ ████████ ███████ █████████████ █████████████ █████ ██ ███ █████ ████████ ██ ██████████ ███ ███ ██████ ██ █████ ████████████ ██ ███ ████ █████████ █████████ ████████
a
Rain clouds above ████████ ███████ ███ ██████ ██ █████████ ████ ████ ██████ █████ ██████████ ████████
Incorrect. This just tells us about the baseline levels of oxygen-18 in clouds above different regions. It doesn't explain why the clouds observed by the scientists did not experience the depletion of their oxygen-18 content as they passed above the Amazon forests.
b
Like the oceans, ████████ ████ ███████ ███ ██████ ██ █████████ ████ ██████ ██ ███ ██████████ █████ █████
Correct. The scientists' surprise rests on the assumption that the rain clouds they were observing stayed the same or were not replenished as they went along, in which case we would certainly expect the oxygen-18 content of those clouds to be depleted. But if tropical rainforests can create new clouds or replenish old ones, this suggests a way that the oxygen-18 content in the clouds under observation could be replenished and brought back to "normal."
c
The amount of ████████ ████ ███ ██████ ████ ███████ ██ ███████ ███ ████ ██ ███ ██████ ██ ████ ██████████ █████████ ██ ███ ██████ ██████ █████ ███ ████████ ██████
Incorrect. Notice that if (C) were true, and the amount of rain falling from the clouds over the Amazon forests is exactly the same as the amount collected over the Atlantic, this would actually suggest that the rain falling over the forests is the "same" rain that is collected over the Atlantic. In other words, this would suggest that the clouds were not replenished as they went along.
As we discussed in our analysis, this would actually deepen the apparent paradox in the stimulus, not resolve it. If the clouds contain a fixed amount of rain, and water molecules containing oxygen-18 fall out of the clouds at a faster rate than regular water molecules, then we're still stuck without an explanation for how the oxygen-18 content of these clouds stays constant.
d
The amount of ████ ████████ ████ ████ ███ ██████████ ████ ███ ██████ ██ ██████ ██████████ ██ ███████ ███ ████ ██ ███ ██████ ██ ████ ██ █████ ███████ ████ ██ ███ ████████ ████ ███ ███████████
Incorrect. Remember that we're expecting an answer choice that says that the rain clouds are replenished as they go along. While this answer choice suggests that some rain can be recycled back into the atmosphere from forest vegetation, it doesn't tell us that this is enough to replenish the clouds. The argument says nothing about river runoffs, so the comparison between the amount of rain recycled back into the atmosphere from forest vegetation and the amount lost in river runoffs doesn't actually tell us anything. There could be very little rain lost in river runoffs, which would mean very little rain is recycled back into the clouds — and in any case, the fact that some rain is lost at all indicates that on net, the clouds are not being "replenished" by the two processes mentioned here. So (D) isn't what we're looking for.
e
Oxygen-18 is not █ ████ █████████ ██ ███ ██████ ██ ████████ ████ ███████ ██ ███ ██████████ █████ █████
Incorrect. Whether or not oxygen-18 is a good indicator of the general effect of tropical rainforests on the atmosphere above them, the conflict in the stimulus is about oxygen-18, not about the general effect of rainforests. So (E) isn't what we're looking for to resolve the apparent paradox about oxygen-18.