PrepTest 116, Section 3, Question 13
Up until about 2 billion years ago, the(41 more words not shown)
Which one of the following,(11 more words not shown)
- AOur atmosphere currently(12 more words not shown)
- BThe liquid water(16 more words not shown)
- CA significant source(18 more words not shown)
- DEvidence suggests that(15 more words not shown)
- EWhen large portions(25 more words not shown)
Reading the argument
What was different about Earth back then?
A sun that dim would leave today's oceans frozen. But the evidence says liquid water existed 3.8 billion years ago, when the sun was still in its dimmer period. We need to accept both facts and look for a difference that lets them fit together.
What we’re looking for
Perhaps the oceans were different in a way that affected freezing. Or perhaps there was another source of heat, such as volcanic activity beneath the water. Those are guesses, not facts we're entitled to assume. They suggest what a useful explanation would need to do: show how water could remain liquid despite receiving less sunlight. For this explanation question, look for a condition that changed between the early Earth and today. The result we'd expect under today's conditions might not apply to the distant past.
Working the answers
- ACorrect
The atmosphere used to hold in more heat. That gives us a difference between the early Earth and the Earth of today: even with less incoming sunlight, more heat could remain instead of escaping.
Think of better insulation. A room can stay warm with less heating if it loses much less heat. Similarly, a dimmer sun needn't produce the same result on an Earth whose atmosphere retains significantly more heat. A helps explain how liquid water could exist 3.8 billion years ago without denying that the same dim sun would freeze today's oceans.
- B
Later freezing and melting don't explain why the water was liquid in the first place. The dim-sun period already included 3.8 billion years ago. B accepts that early liquid water and then tells us what happened afterward, leaving the original puzzle untouched.
Don't read 'up until about 2 billion years ago' as 'only at the moment 2 billion years ago.' If the sun had been dim only later, a story about later freezing might fit differently. Here, the difficult fact is liquid water during the already-dim earlier period.
- C
Another heat source was one of our guesses, but the timing matters. C supplies that heat approximately 2 billion years ago. We need to explain liquid water 3.8 billion years ago. A cause at the later date doesn't establish what kept water liquid at the earlier one.
- D
Some ocean regions remaining frozen into a later period tells us about persistent ice. It doesn't explain how liquid water existed in the much earlier period with the dim sun. Learning that some water stayed frozen isn't the missing reason other water could be liquid.
- E
More ice reflecting more heat would tend to keep an icy Earth from absorbing that heat. The choice doesn't establish a past-versus-present difference that explains the early liquid water. It gives us no reason that less incoming sunlight could still leave water liquid at the time in question.
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