PrepTest 131, Section 2, Question 25
About 3 billion years ago, the Sun was(104 more words not shown)
Which one of the following,(5 more words not shown)
- ASufficient heat to(20 more words not shown)
- BGeological studies indicate(17 more words not shown)
- CGeological evidence indicates(22 more words not shown)
- DThe increase in(20 more words not shown)
- EBecause the distance(28 more words not shown)
Reading the argument
The Sun was dimmer, yet the oceans weren't frozen. Something about conditions 3 billion years ago must have differed from conditions today. My first thought is another source of heat: perhaps volcanic activity or lava beneath the ocean helped keep the water liquid.
The passage then supplies a constraint on that idea. It says Earth could have retained enough heat only if greenhouse-gas levels were higher. So we are given the need for more greenhouse gases; a volcanic explanation by itself won't replace that premise.
What we’re looking for
But why carbon dioxide specifically? The passage names methane as another gas that traps heat. More greenhouse gases could mean more methane rather than a significant increase in carbon dioxide. That's where the conclusion goes beyond the evidence. For a weakener, I'd look for support for another greenhouse gas accounting for the heat retention.
Working the answers
- A
This rules out the volcanic route I initially considered. It supports the stated need to look beyond geological heat rather than challenging it. More importantly, it doesn't distinguish carbon dioxide from methane or other greenhouse gases. It gives no reason to doubt the conclusion about carbon dioxide in particular.
- BCorrect
Much less methane today means there was much more methane then. That supplies a plausible alternative source of the greater greenhouse effect: methane could have helped keep the oceans liquid without requiring the claimed significant increase in carbon dioxide. B doesn't prove methane was the whole explanation or that carbon dioxide stayed constant. It weakens the move from more greenhouse gases generally to more carbon dioxide specifically.
- C
More dissolved minerals might initially suggest another way to keep oceans liquid: a substantially lower freezing point. But the answer explicitly says the minerals weren't enough to alter the freezing points significantly. That qualification removes the significant change this alternative would need. C doesn't supply a useful reason to reject the carbon-dioxide conclusion.
- D
The increasing complexity of life is a different development. The argument concerns how liquid oceans were possible with a dimmer Sun and whether that establishes higher carbon-dioxide levels. A correlation with the development of life doesn't answer that question.
- E
More radiation reaching Earth as the Sun brightened doesn't identify what kept the ancient oceans liquid. Nor does it tell us which greenhouse gas was more abundant then. E is consistent with the stated difference in solar conditions and leaves the inference about carbon dioxide untouched.
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