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PrepTest 73, Logical Reasoning 2, Question 17


Question 17, when a question asks you to weaken an argument, it's a weakened question, which means we have to attack the argument's assumptions. But in order to find the assumptions, we have to understand the argument, which means finding its conclusion and finding its evidence. The conclusion here comes in the middle, the soot itself probably does not cause this ailment.

We need the first piece of evidence to know what type of ailment they're talking about. So there has been a positive correlation found between the amount of soot in the atmosphere of cities and the frequency of a certain ailment among those cities' populations. So in areas that have a lot of soot, you also tend to have a lot of this ailment.

And the conclusion is that soot probably doesn't cause the ailment. The direct evidence for that comes from the end, following that nice evidence keyword, since. Since in cities where there are large amounts of soot in the air, there is usually also high concentrations of many other air pollutants. Now this argument is a causal argument, but with a little bit of a twist.

Usually, the LSAT gives us causal arguments in which the author is trying to prove that one thing caused another thing on the basis of evidence that the two things are correlated. In this argument, the author is trying to argue against a causal connection. Yes, there is a correlation, but it's probably not causal because there's something else that's around, other pollutants.

So there are two major assumptions that we need in order for this argument to work. The first would be that the other pollutants are correlated with the ailment as strong or more strongly than the soot is. And there can't be anything else standing in the way of these other pollutants from causing this ailment, anything about the ailment or anything about pollution that would block the cause.

So we're gonna look for an answer choice that will attack one of these assumptions when we go to the answer choices. So answer choice A, in cities where there are high concentrations of many air pollutants but little if any soot in the air, the frequency of the ailment is just as high, on average, as it is in cities where there are large amounts of soot in the air.

That would actually help the argument. It would support that assumption that the other pollutants correlation is just as strong as soot's is. So this says in places where you have the pollutant and you don't have soot, you tend to still get the thing. So that suggests that the ailment is being caused by the other pollutants, not the soot.

So answer choice A is the opposite of what we want. It would strengthen, not weaken. Answer choice B, if the ailment rarely occurs except in cities in which there are large amounts of soot in the air. And that's already enough to get rid of this answer because we don't know that the ailment rarely occurs except in cities where there are large amounts of soot in the air.

So this is a conditional statement built off of a state of events that we don't know is happening, that if ruins it. Answer choice C, in each of the cities where there are large amounts of soot in the air but little other air pollution, the frequency of the ailment is at least as high as it is anywhere else. Now this answer is the opposite of A.

It attacks that first assumption because it shows that soot remains positively correlated with the ailment even when the other pollutants aren't there. So soot is actually correlated better than the other pollutants are. So C hurts the argument by taking out the assumption, it's our answer. D and E, D would strengthen the argument by further establishing that the other pollutants could cause the ailment instead of soot, though through some sort of combination of pollutants, still it wouldn't weaken.

And then answer choice E would also strengthen by providing yet another thing that's not soot that could be causing this ailment. Their argument is it's not soot, and this says, well, there are other forms of pollution. So not just air pollution, there's other types of pollution that could cause it, too.

So soot gets off the hook, and we're trying to put soot on the hook. That's why answer choice C is the answer.

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