## June 2007, Logical Reasoning 2, Question 11

### Transcript

Question 11. When a question asked what the argument depends on assuming you're dealing with a necessary assumption question. Necessary assumption questions asked you to find something that the argument depends upon, that hasn't been stated, but that if it isn't true, the argument will fall apart.

It's something the argument needs. In order to find what the argument needs, you need to understand it which means breaking the argument down into its component parts, its conclusion and its evidence. The conclusion the argument is found right at the end. Mercury levels in fish are higher now than they were 100 years ago.

The evidence for that is the results of a recent study. They did a study on the feathers of sea birds. They compared seabird feathers from the 1800s and seabird feathers from today. And they found that the ones in the past only had half as much mercury in them as the ones from living bird. Now, the next piece of evidence explains why that study is relevant.

Since, as an evidence keyword, since mercury that accumulates in a seabirds, feathers as the feathers grow is derived from fish eaten by the bird. Once you understand the argument, take a moment to think about what it assumes. This argument assumes a couple of things. The first two things that it assumes relate to the mercury in the feathers. For this argument to work, we need to believe that there's nothing that's artificially reducing the mercury in the feathers of the stuffed birds.

And nothing that's artificially increasing the mercury in the living birds. And by artificially I just mean not involving the saltwater fish. To some other source of mercury reduction or mercury increase, we have to believe those don't exist because if it does exist, this evidence isn't going to prove anything. The other assumption that this argument has is that the birds in question are good representatives of birds of their species.

There's nothing special about the birds that they stuffed or nothing specials about the birds today. That would mean that they aren't very good test subjects for this kind of study. So now that we know what the assumptions are, we go into the answer choices looking for one of them. Answer choice A.

Answer choice A actually hurts to the argument. If the proportion of fish in modern seabird's diets is more than the proportion of fish and old seabird diets, then maybe the increasing mercury is coming from the fact that they eat more fish. It's not that there's more mercury in the fish, is that they eat more fish and so they get more of the mercury.

So something that weakens the argument would never be a necessary assumption in it. So it's not A, go to B. Now, B may seem plausible. Sure, mercury probably comes from pollution in the ocean. But this argument doesn't require any particular source for the mercury.

Sure, pollution is a reasonable source for the mercury, but it could also be natural sources. Some sort of underwater mercury geyser. Something new in the fish's diet that gets mercury into it. It doesn't have to be any particular thing for this argument to work, because this argument is just that it's gone up.

It doesn't matter what the source of the increase is. So answer choice C. Now, this answer provides a reason why you might find mercury in seabird feathers to begin with. But it's not a necessary assumption in the argument. The argument doesn't require believing that birds need mercury in their feathers in order to believe that there's more mercury in their feathers than there used to be.

So answer choice C is not our answer, go to D. This answer, like answer choice A, would actually hurt the argument. This is opposed to the assumption that there's nothing special about the birds. If it were to turn out that the stuffed seabirds whose feathers were tested for mercury weren't fully grown. Then, maybe the reason that there's more mercury in modern birds that we tested is that they're just older.

So it's not that there's more mercury in the environment, it's just that the longer you're in the environment, the more mercury builds up. These were young birds, so they didn't have very much mercury. So that means the next choice is our answered by default, glanced down at it. The process used to preserve their feathers didn't substantially decrease the amount of mercury in their feathers.

This is something we need to assume, cuz remember, the first assumption was there was nothing that got rid of mercury from the old birds. So if we were to find out the preservation process drains mercury from the feathers, then this argument won't work. So the argument requires assuming that the stuffing in preserving process doesn't drain the mercury from the feathers as answer choice E says.

So the answer is E.

Read full transcript