PrepTest 117, Section 3, Question 24
Science journalist: Brown dwarfs are celestial objects with(92 more words not shown)
Which one of the following(9 more words not shown)
- AAny celestial object(14 more words not shown)
- BAny celestial object(16 more words not shown)
- CNo celestial object(11 more words not shown)
- DNo celestial object(11 more words not shown)
- ENo celestial object(11 more words not shown)
Reading the argument
Keep the kinds of objects separate. A brown dwarf is smaller than a star, but the passage doesn't treat it as a small star.
That distinction matters because the lithium story changes with the kind of object. Stars at least as massive as the Sun retain lithium: their internal mixing is incomplete. Stars smaller than the Sun consume their lithium. Brown dwarfs have a different problem: their nuclear furnaces aren't fully functional, so they cannot consume their lithium.
So don't carry the rule about smaller stars over to brown dwarfs just because both have less mass than the Sun. The passage specifically tells us why the brown dwarf's lithium remains. These are facts to organize, not an argument we need to attack.
What we’re looking for
The question asks what is most strongly supported. Stay with what these facts establish. An answer about every celestial object needs support covering that range; we shouldn't fill in missing facts about planets, comets, or other objects just because an answer sounds scientific.
Working the answers
- A
We know that stars with less mass than the Sun lack lithium. That doesn't establish that every object lacking lithium must be one of those stars. What about objects the passage hasn't described? An object could lack lithium for some other reason. The answer takes a fact about one kind of object and treats it as a complete list of all the possibilities.
- B
Incomplete mixing explains the remaining lithium in certain stars. It isn't the only explanation given. Brown dwarfs also retain lithium, but the passage attributes that to a furnace that isn't fully functional. B incorrectly turns the explanation for one group into an explanation for every object with lithium.
- CCorrect
A brown dwarf retains its lithium because it cannot consume it. An object with no lithium therefore cannot be a brown dwarf, according to the description we're given.
Think of lithium as a feature that this account requires a brown dwarf to have. If the feature is missing, that classification won't fit. We don't have to identify what the object actually is; ruling out brown dwarf is enough.
- D
Brown dwarfs themselves show why this fails. They have less mass than stars, so they have less mass than the Sun, and they retain lithium. D says no object can combine those two features, even though the passage gives us a category that does. The rule about smaller stars doesn't cover every smaller object.
- E
Planets have less mass than brown dwarfs, but the passage doesn't tell us whether planets have atmospheric lithium. Nor does it give a lithium rule for every other less massive object. We cannot extend the account of brown dwarfs and stars to everything below that size.
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