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This page shows a recording of a live class. We're working hard to create our standard, concise explanation videos for the questions in this PrepTest. Thank you for your patience!

This is a weakening question: Which one of the following, if true, would cast doubt on the experimenters’ conclusion?

Our stimulus begins with a definition of nuclear fusion; nuclear fusion is a process where nuclei fuse and release energy. We then learn that this process creates a by-product; helium-4 gas. Having established this context, we now learn about an experiment involving “heavy” water. I don’t know about you, but I have no idea what “heavy” water is; luckily the LSAT is about our reasoning skills and not chemistry. We learn that the water is contained in a sealed flask within an air-filled chamber, and that after the experiment some Helium-4 was found in the chamber. The people running the experiment concluded that there must have been nuclear fusion that happened. Since this is a weakening question involving a hypothesis, we should look for an alternate hypothesis in the answer choices, an alternate possible explanation for why there was helium-4 in the chamber. Let’s see what we get:

Answer Choice (A) The researcher’s explanation is entirely compatible with this.

Answer Choice (B) But was it fusion that produced the helium-4?

Correct Answer Choice (C) We are told the chamber was air-filled, so we would therefore expect that level of helium-4. This is a much better explanation than that nuclear fusion occurred!

Answer Choice (D) But it was helium-4 they found. If anything this suggests the helium-4 was recently produced which might support the fusion hypothesis.

Answer Choice (E) Cool, but we weren’t told anything about heat; for all we know there was a large release of heat and our experiment runners hypothesis is entirely correct.


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Here we have a weakening question, indicated by: Which one of the following, if true, would tend to invalidate use of the ratings for the agency’s purpose?

So we’ve got a government rating system where the highest ranked airlines are the ones with the lowest proportion of late flights. So the number one airline would be the one whose ratio of total flights to late flights is the lowest. From the question stem, we know we want to weaken the agency’s reasoning for why this rating would be good for their purpose. We’re next told what this purpose is; the agency thinks the rating can establish an objective measure of the efficiency of the airlines personnel in meeting flight schedules. So the conclusion we want to weaken here is that the proportion of late flights actually represents how efficient the staff of each airline are. We can think of this as hypothesis; the agency believes the reason airlines have more or less late flights is the efficiency of their staff. A good answer choice will be one which introduces an alternate explanation for why airlines are late. On to the answers:

Answer Choice (A) Ok, but this wouldn’t affect whether the staff are what determine flights being late.

Correct Answer Choice (B) If flights are often made late by weather, and some airlines are more affected than others, then it is unlikely the rating would be representative of staff performance because a lot of the time it’s out of their control.

Answer Choice (C) This gives us a reason why being late isn’t always the end of the world, but has no bearing on whether the lateness of flights reflects staff performance.

Answer Choice (D) This is totally compatible with what we want to weaken.

Answer Choice (E) Who cares how we define a late flight? Our interest is in whether lateness represents inefficiency on the part of flight staff.


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This is a weakening question, as our stem asks: Which one of the following, if true, would most weaken the argument?

Our stimulus begins the claim that learning how to build a nest is important for the breeding success for birds. As an example of this claim’s truth, we’re told about a study where blackbirds were less successful breeding while nesting than older birds or even themselves are a year later. So there is a correlation between nest building and high breeding success, got it. To further strengthen the conclusion, the argument eliminates the alternate explanation that it is just about how large and strong the birds are, because they are fully grown once they leave their parents nest. The stimulus ends by reaffirming its conclusion that nesting benefits breeding success. Since this is a weakening question, we want to find an answer that undermines this hypothesis that the higher breeding rates are because of the nesting. Let’s see our options:

Answer Choice (A) Who cares? They could build really nice nests and it still be true that nesting experience benefits breeding success.

Correct Answer Choice (B) This gives an alternate explanation for the increase in breeding success; it’s not their nesting experience, but their experience of attempting to breed, which leads to an increased success rate over successive years.

Answer Choice (C) This would support the conclusion that nesting experience contributes to breeding success.

Answer Choice (D) So just as the argument stated, it’s not because of size and strength; this does not weaken the argument at all.

Answer Choice (E) Ok but we are interested in the increase after they started to nest; who cares about the birds that didn’t make it (no offense to any avian 7Sagers)?


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This is a weakening question: Which one of the following, if true, would weaken the argument?

Here we have a stimulus which begins with the conclusion; our author declares that there is no point in requiring scientists’ work be officially confirmed before being published. This is because there is already an unofficial confirmation system; the ability of other scientists to replicate results. Poor scientific work will be revealed when other scientists fail to replicate it, and therefore will cause no harm. So we don’t need to vet what gets published because scientists will naturally figure out whether a publication is flawed and harmful. Interesting! Our job is to weaken this conclusion: we want ACs which suggest that the official confirmation is important and not redundant! Let’s see what we get:

Correct Answer Choice (A) Bingo! If replication can take years to occur, then that is years a flawed study could be out there causing harm.

Answer Choice (B) This strengthens the conclusion, by providing another vetting system which could catch issues regardless of official confirmation.

Answer Choice (C) This also supports that replication will be able to find errors if unofficially confirmed work is published.

Answer Choice (D) This does nothing to undermine the author’s conclusion.

Answer Choice (E) Cool! But what does this have to do with whether official confirmation is important!


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This is a flaw question, and we know that because of the question stem: A flaw in the reasoning in the argument above is that this argument…

We’re told that a when liquid from a bottle labeled “vinegar” is added to a box labeled “baking soda”, it does not fizz. Usually, when baking soda and acidic liquid (like vinegar) is combine, fizzing occurs. Because of this, the author concludes that the bottle was mislabeled. However, is this the only thing mislabeled? We don’t have enough evidence to rule out that the box was not mislabeled. We also can’t rule out that the vinegar is just gone bad and that’s why it’s not reacting.

Answer Choice (A) is not descriptively accurate; this is exactly what he’s suggesting.

Correct Answer Choice (B) is descriptively accurate and it’s the flaw. We already listed out two possibilities it could have been that the stimulus fails to rule out.

Answer Choice (C) is descriptively inaccurate; the use of the term fizz does not change.

Answer Choice (D) is descriptively accurate (it’s not entirely that principles can only be tested in labs, but let’s grant that it is); however, this is not a scientific principle.

Answer Choice (E) is descriptively inaccurate - the argument does not saying anything about the intention to deceive.


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This is a flaw question, and we know that because of the question stem: As a rebuttal of Giselle’s argument, Antoine’s response is ineffective because

Giselle’s conclusion is that the government should raise sales tax on gas. Why? Well, because the government needs to make sure that the public consumes less petrol and when things cost more, people buy less of it.

Antoine’s conclusion is that the government should not raise the tax on gas because it’s not fair to gas users. Instead, if gas prices are to be increased, it should be in such a way that it’s a burden to everyone - not just the gas users.

Giselle’s argument is definitely weak - there is no firm evidence that taxes on gas should be increased. There are plenty of other ways to make people stop using gas, including giving people incentives or just restricting the supply of gas (which will inflate the price of gas itself, making the gas extremely expensive). However, Antoine’s government doesn’t effectively address Giselle’s argument as much as it disagrees with her conclusion and fails to address the validity of her conclusion. In addition, he talks about increasing the government’s revenue; that’s not what Giselle’s argument is about. Her argument surrounds cutting gas consumption, not about getting the government more money.

Correct Answer Choice (A) works because it’s descriptively accurate and it points out where Antoine fails to address Giselle’s argument - on the LSAT, disagreeing with someone’s conclusion involves demonstrating that their premised doesn’t allow for their conclusion to be made. Antoine fails to do this.

Answer Choice (B) is descriptively accurate but it’s not a flaw. He’s talking about being fair to users of gasoline; we don’t need to know the number of taxpayers that are not gasoline users.

Answer Choice (C) is descriptively accurate, but it’s now a flaw. While fairness is subjective, he does support his argument by explaining for it’s unfair.

Answer Choice (D) is descriptively inaccurate. Giselle says that the government should increase taxes on gas - it’s okay for Antoine to assume this.

Answer Choice (E) is descriptively inaccurate. He is not making the assumption that the government’s revenues should/can be increased in other ways. He’s argument that, if these taxes are increased, it should raise the government’s revenue in a way that doesn’t burden just the group of users.


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