PrepTest 108, Section 2, Question 20
One of the most important events for modern(78 more words not shown)
Which one of the following(8 more words not shown)
- AComets tend to(12 more words not shown)
- BIf Jupiter's lower(16 more words not shown)
- CA larger explosion(20 more words not shown)
- DThe post-collision analysis(12 more words not shown)
- EThe deeper the(15 more words not shown)
Reading the argument
No signs of water. Does that mean no water was involved, or could the water have escaped detection?
The observations come from formations left by the comet fragments' collisions with Jupiter. The author uses the absence of water signs to infer that no water was involved, then draws two further conclusions: the fragments didn't reach the lower atmosphere, and the comet was rock rather than ice.
I want the missing connections explained. Why would reaching the lower atmosphere leave evidence of water? And if water had been involved, would our observations necessarily have picked it up? We can't simply treat an unseen thing as an absent thing without some reason to expect it would have been visible in the evidence.
What we’re looking for
The question asks for a necessary assumption. My first expectation is a connection between water's presence and detectable signs of it. The author needs that kind of link to use this observation to rule something out. We should also keep track of which part of the argument each answer addresses; there are several conclusions being drawn from the same observation.
Working the answers
- A
A general tendency in comets and asteroids doesn't establish what happened in this particular collision. The argument doesn't need most comets to be icy or most asteroids to be rocky to draw its inference from the formations. It needs a connection between what happened and what the resulting evidence would show.
- BCorrect
If the fragments had reached Jupiter's lower atmosphere, the formations would have shown signs of water. That is the link needed for the conclusion about how far the fragments penetrated.
Suppose instead that the fragments could reach the lower atmosphere without leaving signs of water in the formations. Then the observation would look just as it does now even though that conclusion was false. The missing signs couldn't rule out penetration. B prevents that possibility by saying penetration would have produced the very evidence we don't see.
- C
We aren't told that the explosion's size is evidence of the comet's composition. The author relies on the absence of water signs. A difference between the sizes of rock and ice collisions isn't required for that reasoning, and the passage gives no measured explosion size to combine with C.
- D
The formations could differ in many respects while all showing no signs of water. Sharing that one absence doesn't require identical composition. The argument needs the reported absence to be informative; it doesn't need every formation to be exactly the same.
- E
This compares how hard explosions at different depths are to detect. The argument doesn't need such a gradual relationship. Even if deeper explosions were equally easy to detect, the author's proposed connection between lower-atmosphere penetration and signs of water could still operate. E doesn't supply the specific connection required here.
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