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PrepTest 73, Logical Reasoning 2, Question 15


Question 15, when a question asks you to explain a surprising fact, then it is a paradox question. Paradox questions present us with a scenario in which two things don't seem to match. On the one hand, one thing's true, on the other hand, something else is true. It doesn't seem like they should both be true.

It's weird, and you're going to explain away the weirdness. Or rather, you're gonna pick an answer that explains away the weirdness. One of the answers will be a new fact which explains why it is that these two things do coexist. On this question, the weird situation is, well, first off, there's a type of dragonfly, it's endangered, it lives in the wetlands and it's larva can only live in the water where they are subject to predation by red devil crayfish.

But on the other hand, those dragonfly populations do better in areas where the crayfish are than they do in areas where they're not. So our paradox is, these crayfish eat the babies of the dragonflies. And yet, dragonflies do better in areas where there are these baby eaters. Generally speaking, having something eat your offspring is not conducive to survival.

We're gonna look for an answer choice that'll explain why it is that the crayfish help them out. No, it won't contradict anything we've already heard. So we're not gonna find out, it turns out they don't eat the babies. They definitely eat the larvae. The correct answer choice will just explain some sort of benefit that they provide.

So answer choice A, red devil crayfish dig chambers that remain filled with water even when the surrounding wetlands dry up. So if you look back to what we heard about the dragonflies, they can survive only in water. So if the red devil crayfish dig chambers that remain filled up with water even when the rest of the wetlands are dry, that would be to the advantage of the dragonflies.

It's giving their larva a place to live. Sure, the red devil crayfish may be charging rent by eating some of them, but without the red devil crayfish there wouldn't be any larvae at all, they'd dry up. So A gives us our way out, it explains how even though the crayfish eat them, the crayfish can benefit them too.

So then what's wrong with the other answers? Well, answer choice B, red devil crayfish present no threat to adult dragonflies. This might seem like a good answer because it exempts the adult dragonflies from the crayfish's effect. But the paradox isn't just the dragonflies are able to survive in spite of the crayfish.

They actually do better when there are crayfish. That's why we're trying to understand, how is it that they do better? Not eating them would be not affecting them. Why would that make them better than otherwise? So answer choice C. C actually makes the paradox worse.

It negates something that could have been the answer. So one way that the crayfish could help out the dragonflies would be if they killed off some other predator of the dragonflies. An enemy of my enemy is my friend sort of situation. But this answer choice says specifically they don't do that. So they are not helping out the dragonflies.

And then answer choice D, red devil crayfish are found in many more locations than the dragonflies are. This won't help either way, the paradox only takes place when they overlap. When they overlap, the dragonflies do better. So places where there are crayfish but no dragonflies won't do anything to the paradox.

And then answer choice E, all it does is show us that the crayfish don't need the dragonflies because they live even if the dragonflies die out. That doesn't explain why the dragonflies do better when the crayfish are there. So the answer was answer choice A

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