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.
Strategy: Most Strongly Supported
We're asked to come up with an inference about an individual rain cloud. The places to look are probably the second and third sentences, which tell us about rain clouds and rainfall in general — but for a question stem as general as this, it makes sense to go to the answer choices and proceed by process of elimination.
20.
Which one of the following ██████████ █████ ██ ██████████ ████ █████ ██ █████████ ██ ███ ████████
a
Once it is ██████ ████ ███ █████████ ███ ████ █████ ████████ ████ ████████ ██████ ████ ██████████
Correct. The stimulus tells us that in a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing regular oxygen. Since there are fewer oxygen-18-containing molecules than regular oxygen-containing molecules in a rain cloud, it is strongly supported to say that a rain cloud at the site of its original formation over the Atlantic, before any rainfall, will contain more ordinary oxygen than oxygen-18.
b
Once it has ██████ ████ ███ ███████ ███ ████ █████ ████████ █ ███████████████████ ██████████ ██ ██████████
Incorrect. We're told that the oxygen-18 content of these clouds remains "fairly constant" as they pass over the Amazon, and we have no reason to think that the "constant" level of oxygen-18 — i.e., the level of oxygen-18 these clouds had when they formed — was greater than normal. So we don't have any reason to think that once a cloud has passed over the Amazon, its percentage of oxygen-18 will somehow have become greater than normal.
c
The cloud’s rainfall ████████ ████ █████████ ████ ████████ ███████
Incorrect. We're told that in rainfall, a higher proportion of water molecules containing oxygen-18 will fall to earth than of water molecules containing regular oxygen. But remember that there are fewer water molecules containing oxygen-18 in a rain cloud to begin with, which means that even if a higher proportion of these molecules fall compared to water molecules containing regular oxygen, there could still be more regular oxygen than oxygen-18 in the rainfall overall.
As a silly example, suppose there are 10 molecules in a raincloud containing oxygen-18, and 100 containing regular oxygen. If 5 oxygen-18-containing molecules descend in the rainfall, and 10 of the regular oxygen-containing molecules, that's 50% of the oxygen-18-containing molecules, compared to 10% of the regular ones. And yet in raw terms, there is still more regular oxygen than oxygen-18 descending in the rainfall.
d
During a rainfall, ███ █████ ████ █████████ ███ ████ ██████████ ██ ███ ████████ ██████ ██ ██ ███ ██████████
Incorrect. (D) actually contradicts the stimulus. If a raincloud contains a smaller overall amount of oxygen-18 than of regular oxygen to begin with, and loses a higher proportion of its oxygen-18 than of regular oxygen with each rainfall, then it's impossible for the "same percentage" of regular oxygen as of oxygen-18 to be surrendered during a rainfall.
e
During a rainfall, ███ █████ ████ █████████ ████ ██ ███ █████████ ████ ██ ████████
Incorrect. We know that the proportion of oxygen-18 surrendered by a cloud during a rainfall is higher than the proportion of regular oxygen. But we have no idea how high that proportion actually is in numerical terms, let alone whether it is more than 50%, as (E) would require.