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#8 - This boulder is volcanic in origin and yet the rest

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I was wondering if someone could explain how to go about selecting answer choice D. I initially chose A, thinking that such information would weaken the fact that the glacier could not move the boulder that far. The second time around I chose C, even though answer choices B and C look very similar. It makes sense after looking at it for awhile that D provides an alternative explanation. However, I am not convinced of my explanation.

Thank you!
Charlie Melman
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Hi L,

This is a classic LSAT stimulus, in that it gives one possible explanation for a set of facts, but does not consider any of an infinite set of alternative explanations. The correct answer choice is going to give us a fact or explanation that highlights this.

Answer choice (A): "Most" on the LSAT means that the thing in question has a 51-100% chance of occurring. Even if what answer choice (A) says is true, it could be that 49% of glaciers are moved hundreds of miles. That's barely a weakener.

Answer choice (B): The stimulus says the rock was moved hundreds of miles. If the closest source of rock is 50 miles away, then it's entirely possible that there could be a farther source that's hundreds of miles away.

Answer choice (C): Same as answer choice (B). The south/north distinction does not matter, since both (C) and (B) could be true while the stimulus is true.

Answer choice (D): This is correct. If the glaciers were moving southward to the site mentioned in the stimulus, they had to come from north of the site. If there are no sources of volcanic rock north of the site, then the "geological birthplace" of the rock couldn't have been north of the site, and thus the glacier couldn't have carried it down.

Answer choice (E): Irrelevant. We don't care about other volcanic boulders, we care about the geological origin of the boulder in question.

Hope this makes sense!
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Hi, I am trying to make a good prephrase for this question, but I am not sure how to go about it. I came up with something along the lines of "It is not necessarily true that the boulder deposited here by a glacier." The answer choices all sounded very similar to me. In general, I have a harder time with prephrases for science-related topics. Do you have any advice about how to attack such questions?
Adam Tyson
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I think your prephrase is on the right track, bk1111. Since the conclusion is that a glacier deposited the boulder, you want to pick an answer that calls that into question. Maybe the boulder got here some other way? If we had an answer that tracked with that, we'd be happy - something like "volcanoes often eject boulders with such force that they land far away from their source" or "this boulder shows markings on it consistent with its having been moved here by machines made by humans", or even "this boulder's mineral composition shows that it originated on another planet and could have landed on this spot from outer space".

Unfortunately, none of our answer choices suggests any alternate scenarios like those (Alternate causes, perhaps? Might this stimulus have causal reasoning?) so we have to look elsewhere.

Consider that most arguments on the LSAT are flawed, and that all flaws can be described one way or another as being based on a possibly flawed assumption. When we weaken a question, we are challenging the author's assumptions. When we strengthen, we are supporting those assumptions. So what assumption did our author make here? in order to claim that the volcanic boulder arrived at this spot by southward glacial movement, he must have assumed that somewhere to the north of this spot is a source of volcanic rock! To weaken the claim, attack that assumption - look for something that says "oh no there isn't!" If there is no source north of here, then his claim that a southward-moving glacier brought it here isn't just weakened, it's pretty well destroyed. There's answer D giving us exactly that.

Answer A sure does look like it would weaken the argument to me, but not by much, as our explanation says. Just because most boulders don't get moved hundreds of miles from their point of origin, that doesn't mean that all don't, or that this one didn't. I kept this as a contender, but only until something that did more to weaken the argument came along, and D fit the bill by making the claim not just a little weaker, but nearly impossible.

In prephrasing, consider those two approaches, which are very similar to the way we look at Assumption questions (Supporters and Defenders). On the one hand, consider alternatives to the claimed relationship, and on the other, consider attacking the stated claim directly by making it less likely to be viable.

Keep at it, you'll get there!
Adam M. Tyson
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Am I misunderstanding the stimulus? D doesn't make sense to me. If the glaciers were moving south, then to weaken it wouldn't you say there are no glaciers south?
Thank you
Eric Ockert
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Well, the glaciers are moving south, which implies they are coming from the north. So, if southward moving glaciers are supposed to be bringing this volcanic rock to its current position, then the source of that volcanic rock would presumably be north of its current location. So if answer choice (D) is true, and there are no geological sources of volcanic rock north of the current location, that would make it very unlikely that glaciers were able to pick this rock up and bring it to its current position.

Hope that helps!
Eric Ockert
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