PrepTest 116

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
3.8 billion years ago — Life and liquid water were present on Earth
2 billion years ago (until) — The sun was 30% dimmer than now
Today — If the sun were 30% dimmer, the oceans would be ice

Answer Anticipation:
As with all Paradox questions, we should start by clearly defining the paradox. Here, it relates to water.

Today, if the sun were 30% dimmer, the oceans would be completely frozen. However, when the sun was 30% dimmer 3.8 billion years ago (until 2 billion years ago), there was liquid water present on Earth. How could this be?

As with many Paradox questions, the unexpected situation here deals with a comparison—comparing the sun's brightness and its impact on water now and billions of years in the past. To explain the different effect this dimness has, we should look for a difference between the two time periods that can account for it. For example, something about the Earth's atmosphere at the time may have made a dimmer sun still able to heat water above the melting point.

Answer Explanation:
This answer brings up a relevant difference between present—day Earth and Earth 2.8b years ago. If the atmosphere held in a lot more heat 3.8b years ago, that could explain how the Earth was hot enough for ice to melt into water even with a dimmer sun. This answer provides an explanation, so it's correct.

Key Takeaway:
When a Paradox revolves around a comparison, look for an answer that brings up another relevant comparison. If the paradox involves a difference, look for a relevant difference.

Incorrect. This answer just presents a whole bunch of new questions—why was it liquid until then, and why did it freeze after? What happened to life if the liquid water froze? This doesn't resolve the paradox—it just makes things worse.

Incorrect. The paradox is that there wasn't just ice 3.8—2b years ago, but rather liquid water. This answer, in discussing ice from before 2b years ago, doesn't align with that.

Incorrect. This answer is about certain regions" of the ocean, not the whole thing, so this answer is too limited to resolve a paradox about Earth as a whole.

Incorrect. This answer might explain why an ice—covered Earth is cooler than one that isn't covered in ice, but it doesn't explain how there was liquid water at a time when the sun was significantly dimmer. There's also no indication of any time period where the Earth was completely covered in ice.