shifty_eyed wrote:It's not a MUST BE TRUE question, and common sense tells us that water molecules make up the MAJORITY if not all of the cloud. Also, since we know that oxygen-18 is heavier than oxygen-16, it doesn't make much common sense to think that it might be a common element in other gases that might comprise a small portion of the cloud (common sense tells us that heavier elements are less volatile, and therefore less likely to be in gases). Therefore, the answer is supported by the stimulus.
I don't think we have to get into all this.
As already stated, you're looking to support the answer choice, not conclusively prove it. When they tell us that water molecules containing oxygen-18 are rarer than water molecules that contain normal oxygen, that supports the choice they're looking for, namely that there's more normal oxygen in the cloud when formed than oxygen-18. True, it leave open the question Timmy already brought up about there being other types of molecules that might contain the oxygen-18, but we don't know for sure whether there are any other such molecules because the argument doesn't tell us. All we know for sure about that is the water molecules containing oxygen-18 are rarer.
I think it we were to swap that premise for the conclusion, and look at it like a strengthen question, it would make more sense. What would support the "conclusion" that the cloud has more normal oxygen than oxygen-18 when formed? A choice that said "the cloud has less water molecules that contain oxygen-18 than water molecules that contain normal oxgyen", would offer support for that "conclusion". In other words, I'm saying that when they ask us to support that choice, they're really asking us to support it as a conclusion for the argument, just like a strenghten question. Except than in a strengthen question, we don't really expect to come up with conclusive proof--but in this kind of question, it's easier to expect the answer choice to be better supported than it is in this question.