PrepTest 158, Section 2, Question 14
Unlike stars, planets do not generate light, but(33 more words not shown)
The reasoning in the argument(17 more words not shown)
- Athis galaxy contains(9 more words not shown)
- Bcelestial objects in(15 more words not shown)
- Ccelestial objects in(11 more words not shown)
- Dthere are numerous(12 more words not shown)
- Eplanets are not(9 more words not shown)
Reading the argument
What tells us that those other objects make light?
Planets don't generate light, and stars do. But the galaxy also contains objects that are neither planets nor stars. That doesn't automatically put those objects on the light-generating side. They might be like planets in this particular respect even though they aren't planets.
Think about something like a moon or an asteroid. We don't need an astronomy lesson about what each actually does. Just ask whether the passage gives us a reason that an object outside both named groups must produce its own light. It doesn't. “Not a planet” isn't the same statement as “generates light.”
What we’re looking for
The flaw question asks for a possibility the author overlooked. I'd say: what if the other objects don't generate light either? That would leave us with nonplanets that provide no example of a light source outside the stars.
Working the answers
- A
This concerns which objects reflect light. Our conclusion concerns which objects generate light. Saying only planets reflect it doesn't establish that every other object produces it. A also doesn't identify the overlooked possibility we need: that nonplanet objects might fail to generate light.
- B
The fraction of the universe's objects located in this galaxy doesn't tell us whether the objects here generate light. We could be examining a tiny fraction or a large fraction and still have the same gap in the reasoning.
- C
Other galaxies could contain light-producing stars. That wouldn't give us a nonstar object in this galaxy that produces light. Both the location and the kind of object matter to the author's conclusion; C doesn't establish the missing case.
- D
Stars and planets could differ in many other respects, but those differences don't settle what the other celestial objects do. We are questioning an inference about light generation. A list of additional ways stars differ from planets doesn't justify that inference.
- ECorrect
Other objects might share the planets' failure to generate light. If so, being a nonplanet wouldn't make something a source of light. The author needs evidence that at least some of the other objects generate light, but gives us only evidence that they aren't planets or stars. E identifies the possibility that this evidence leaves open.
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