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PrepTest 73, Logical Reasoning 2, Question 13


Question 13, when the question asks us for the assumption that an argument relies upon, it is a necessary assumption question. On a necessary assumption question, your job is to find something that the argument hasn't said, but that is required for the evidence to be able to prove the conclusion. Assumptions are essentially unstated premises of the argument.

In order to find them you have to understand the argument, which means understanding its conclusion and its evidence. This argument isn't very long and its conclusion is helpfully flagged by the conclusory keyword so, right there at the end. So any methane in the Martian atmosphere must have been released in the atmosphere relatively recently.

The rest of the argument has two parts. The first is just a little bit of setup information. It's not playing any logical role in the argument, but it tells us that in early 2003, some scientists found methane in the atmosphere of Mars. And then the main piece of evidence for the conclusion is since methane is a fragile compound that falls apart when hit by UV radiation in sunlight.

That's the evidence that proves or attempts to prove that the methane that's in the Martian atmosphere must have been released relatively recently. The main assumption of this argument has to do with exceptions. We know that methane breaks down quickly when hit with sunlight. So that would seem like a sealed deal. Sunlight plus methane equals no more methane.

So is there any way that the methane in the atmosphere might not get exposed to sunlight? If there is, then this evidence wouldn't prove that the methane has to be relatively recent, if there's somewhere in the atmosphere that the methane can hide. I don't know how, I don't need to know how, however it is, then this argument wouldn't work.

So, we would say that the argument assumes that there isn't any way that the methane in Mars' atmosphere could somehow escape exposure to sunlight. Because once it's hit, it goes away very quickly. So that's what we're going to be looking for in the answer choices. Answer choice A, Mars had no methane prior to 2003. This is a trick meant to get you thinking about the initial scenario or the initial setup information we were given.

The argument is really only about the methane that is in the atmosphere right now. Past methane doesn't matter unless it's methane that somehow managed to escape sunlight. Now A is not the answer. Answer choice B, all the methane in the Martian atmosphere is eventually exposed to sunlight.

This is a different way of saying of what we said just a second ago. There's no way for methane to avoid sunlight. Eventually it is exposed to it, and then when it with that sunlight, it disappears. Answer choice B is what we have to believe for this argument to work. So the three remaining answer choices are things that we don't have to believe. C is about whether we can detect methane.

So this is again trying to use that setup information to confuse you. We know that 2003 is when some scientists found methane. We don't need to believe anything about the detection of methane to believe that methane disappears quickly in the atmosphere of Mars. Answer choice D is again about the detection of methane. Again, they're trying to confuse us with that initial information.

That initial information plays no logical role in this argument, so nothing needs to be true about it either way. And then finally E, E brings in the Earth's atmosphere. What happens on earth is entirely separate from this argument. You could say is millions and millions of miles from this argument because this argument is all about Mars.

So the answer was answer choice B.

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