Critical Reasoning

Between 1980 and 2000 sea otter population of the Aleutian Islands declined precipitously. There were no signs of disease or malnutrition, so there was probably an increase in the number of otters being eat by predators. Orcas will eat otters when seals, their normal prey, are unavailable, and the Aleutian Islands seal population declined dramatically in the 1980s. Therefore, orcas were most likely the immediate cause of the otter population decline.

Which of the follow, if true, most strengthens the argument?
a) The population of sea urchins, the main food of sea otters, has increased since the sea otter population declined
b) Seals do not eat sea otters, nor do they compete with sea otters for food.
c) Most of the surviving sea otters live in a bay that is inaccessible to orcas.
d) The population of orcas in the Aleutian Islands has declined since the 1980s
e) An increase in commercial fishing near the Aleutian Islands in the 1980s caused a slight decline in the population of the fish that seals use for food.

The answer is C. Which I just don't understand, I feel to come to that conclusion, you have to make up an entire back story which you're not supposed to do.

Can someone walk me through another mindset? Please and thank you.

In a way you have the right idea. Making up crazy convoluted stories is one way people end up choosing wrong answers to GMAT critical reasoning questions.

"Lemme see. Police like coffee and there are usually coffee shops in airports. So likely there is more crime in the town without the airport, and that's supports the thesis that airports reduce crime!!!! Yeah, that's the ticket!!!!"

That's the ticket to a wrong answer.

At the same time, you can, you even have to, make logical connections to get the right answers to CR questions.

For instance, if there is a question about why an umbrella company went out of business and one of the answer choices says that the county has experienced a three year drought during which there have been only ten days of rain, it's not making up a story to conclude that the company went of out business because people stopped buying umbrellas. It's very simple and direct. People use umbrellas primarily to remain dry when it's raining. No rain - no need for umbrellas.

In the question above, there is being discussed a hypothesis, that the reason for the otter population decline is that orcas were eating the otters. The question asks for information that backs up that hypothesis.

A) This is a logical result of the otter population decline, but clearly does not support that hypothesis of why the otter population decline happened.

B) This answer choice is one that requires making up a story, or finding some connection that is not clearly there. You could start saying something like, "The seal population declined maybe because their usual food source was not available, but seals don't eat otters and the only other animals around are orcas, and maybe orcas eat the same things that seals eat. So if the seals are not eating the otters, it must be that the orcas are eating the otters!!!!!"

Uh, not so fast.

C) This one is way more direct. It's like the results of a scientific experiment. Let's see what happens if we put otters where there are no orcas. Otters + Orcas = No Otters. Otters + 0 Orcas = Otters. Does that relationship prove that the orcas are eating the otters? No, but it does suggest that the orcas have something to do with the otter population decline. So while it does not prove the hypothesis to be without a doubt correct, it does provide some support for it.

D) If anything this weakens the argument, but really it does not matter. Even if there are fewer orcas around than there had been, whatever orcas are around could be hungry and therefore eating the otters.

E) This is about seals rather than orcas and otters. So without a convoluted back story, it's irrelevant.

So while it does not unequivocally prove the argument to be correct, one answer choice and only one answer does lend some credence to the hypothesis. If there were another answer choice that said like, "Orcas in the Aleutian Islands were watched carefully and found to eat only two things, seals and otters, and when the seal population went down they ate more otters than they had eaten before." then that would be the best answer, but as it stands, none of the other answers supports the argument more than does choice C.