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Information and Ideas / Inferences Difficulty: Hard

Many animals, including humans, must sleep, and sleep is known to have a role in everything from healing injuries to encoding information in long-term memory. But some scientists claim that, from an evolutionary standpoint, deep sleep for hours at a time leaves an animal so vulnerable that the known benefits of sleeping seem insufficient to explain why it became so widespread in the animal kingdom. These scientists therefore imply that blank

Which choice most logically completes the text?

prolonged deep sleep is likely advantageous in ways that have yet to be discovered.

most traits perform functions that are hard to understand from an evolutionary standpoint.

it is more important to understand how widespread prolonged deep sleep is than to understand its function.

many traits that provide significant benefits for an animal also likely pose risks to that animal.

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Explanation

Choice A is the best answer. It most logically completes the text. The text says that some scientists can’t explain why prolonged deep sleep is so widespread, given that the known benefits of sleep don’t seem to make up for how vulnerable it leaves an animal. This suggests that prolonged deep sleep probably has unknown evolutionary benefits that make up for the vulnerability.

Choice B is incorrect. It doesn’t logically complete the text. The text only discusses the benefits and risks of one trait: sleep. So there’s no basis for an inference about “most traits.” Choice C is incorrect. It doesn’t logically complete the text. The text says that it is already known that prolonged deep sleep is widespread in the animal kingdom. Rather, what some scientists can’t explain is WHY prolonged deep sleep is so widespread, given that the known benefits of sleep don’t seem to make up for how vulnerable it leaves an animal. Choice D is incorrect. It doesn’t logically complete the text. The text only discusses the benefits and risks of one trait: sleep. So there’s no basis for an inference about “many traits.”