LR Problem Set #2, Q: 165

[moshei24]

Hi,

I'm not really sure as to how the scientist comes to his conclusion that the ferrous material caused the increase in the the algae, such as diatoms. How did he realize that? If the diatoms absorb CO2, then wouldn't that be what causes them to increase?

Then, for how to weaken this question - I see why D works because if there was an increase in diatoms, there should also be an increase in the shells they leave when they die. But C could also weaken the conclusion - it gives a possible alternative cause being that there were other minerals found, so wouldn't they have been able to cause the increase in diatoms as opposed to the cause being the ferrous material?

Unless, the main part of the conclusion isn't that the ferrous material was the cause, but rather the main conclusion is that there was an increase in diatoms with the the cause being ferrous material being a premise. But that isn't the case. It's actually a causal conclusion - the ferrous material caused the increase in diatoms, so both C and D should be able to work. D shows that the stated effect doesn't occur, and C is giving the possibility that there was and alternate cause for the stated effect.

Am I missing something key in this question? If someone has time to explain this question in its entirety so I understand how the scientist got to his conclusion in the first place that the diatoms were the cause, and then to explain why C doesn't weaken the conclusion would be great.

Thanks!

[Justin Eleff]

Remember that the question stem here asks you to pick the answer that MOST SERIOUSLY undermines the hypothesis. So it's certainly possible that more than one answer choice might, if cast in a certain light, function to weaken the scientist's argument. But if we're choosing between (D) and (C), (D) is clearly the better answer.

The scientist theorizes that the ferrous material caused an increase in the population of algae like diatoms. But (D) tells us that diatoms leave evidence of their existence when they die -- namely, they leave these shells -- and there was no corresponding increase in the number of these shells (the amount of this evidence) left behind. So it seems much less likely that there was any increase in the population of the algae, because the evidence that we'd expect to see of such an increase is just not here.

In (C), all we have is "other minerals." They might be alternative cause-type minerals -- that is, it's possible in the abstract that if ferrous material can cause an increase in the population of algae, "other minerals" could too -- but the answer choice does not say anything more about what kinds of minerals (with what kinds of properties) they are. If the same answer choice had said, "in addition to the ferrous material, microscopic quantities of peanut butter were present in the air bubbles," your brain probably wouldn't have assumed that those quantities of peanut butter were an alternative cause for the proliferation of the algae ... and yet, there's nothing in the wording of the answer choice to make these "other minerals" any more likely to have served as an alternative cause than the peanut butter would have been.

So because (D) removes an effect (of algae having increased at all) far more directly and concretely than (C) provides an alternative cause (of "other minerals" being the right stuff to promote the algae increase), (D) is the better answer.

[moshei24]

Okay, thank you, but how does the scientist come to his conclusion in the first place? It's not necessary for the answer, but it would be nice to see how he got there, unless it's just assumed that he got there without an explanation in the question.

[moshei24]

But if (D) wasn't there, (C) would be the correct answer?

Because in general the LSAT doesn't give one answer and one better answer, they just used "most" to cover for questions like these that have two possible answers, but are extremely rare...