PT116.S3.Q13

PrepTest 116 - Section 3 - Question 13

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Up until about 2 billion years ago, the sun was 30 percent dimmer than it is now. ██ ███ ███ ████ ████ ███ ████ ███ ██████ █████ ██ ██████████ ███████ █████████ ██ ██████ █████████ ████████ ████ ███ ██████ █████ ████ ████ ███████ ██ █████ ██ ███ ███████ █████ ████

Structure: Surprising Phenomenon

According to the stimulus, the sun was 30 percent dimmer than it is now until around 2 billion years ago — dim enough that, if the sun were that dim now, the oceans would entirely freeze. But the stimulus presents us with a surprising phenomenon: the fossil record tells us that life and liquid water were both around as early as 3.8 billion years ago, well before the sun got brighter.

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13.

Which one of the following, ██ █████ ████ █████ ██ ███████ ███ ████████ ███████████ █████████ ██████

a

Our atmosphere currently █████ ██ █████████████ ████ ████ ████ ██ ███ ███ ███████ █████ ████
Correct. (A) fits perfectly with what we pre-phrased. If the earth’s atmosphere held in more heat 3.8 billion years ago than it does now, this explains how it could have been warm enough for the oceans not to freeze, even though the sun was significantly dimmer.
64%
b

The liquid water ███████ ███ ███████ █████ ███ █████ ██████ ████ ██ ████ █████ █████ █ ███████ █████ ████
Incorrect. Even if the water somehow froze later, we need to explain how it was liquid to begin with. The only fact we know about what things were like more than 2 billion years ago is that the sun was 30% dimmer than it is now. Given this fact, regardless of what else happened between 3.8 billion years ago and 2 billion years ago, we know that at the 3.8-billion-year mark, the sun was 30% dimmer and yet there was liquid water. We need an explanation for this specific phenomenon, and (B) doesn't provide it.
20%
c

A significant source ██ ████ █████ ████ ███ ███ ███████████ ██ ███ ███████ ██ ███ ██████ █████████████ █ ███████ █████ ████
Incorrect. We don’t want to know about ice sheets melting 2 billion years ago — we want to know how liquid water could be present 3.8 billion years ago. Similar to (B), (C) is trying to distract us by talking about things that happened later than 3.8 billion years ago, without providing an explanation for how there was liquid water at the 3.8-billion-year mark.
12%
d

Evidence suggests that ███████ ███████ ██ █████ ████████ ██████ █████ ████ ████ ████████ ████ █ ███████ █████ ████
Incorrect. Even if this is true, we know that the ocean would be completely frozen if the sun were 30% dimmer now. We want to know why the ocean wasn’t completely frozen 3.8 million years ago. Knowing that parts of the ocean stayed frozen well after 2 billion years ago doesn’t help us.
2%
e

When large portions ██ ███ █████ ███ ████████████ ████ ██ ███ █████ ████ ██ █████████ ███ ███ ████████ ██ ███ █████ ████ ████ ████ ███ █████ ███ ████████████
Incorrect. We don’t know how much of the globe was ice-covered 3.8 billion years ago, just that there was some liquid water around. If we assume that the globe was heavily ice-covered 3.8 billion years ago, (E) would lead us to expect that the earth would be even cooler. So (E) doesn’t provide any explanation for how there was liquid water around 3.8 billion years ago.
2%

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