PrepTest 109, Section 4, Question 5
Astronaut: Any moon, by definition, orbits some planet(16 more words not shown)
The astronaut's conclusion follows logically(8 more words not shown)
- AThere is only(4 more words not shown)
- BEvery moon in(5 more words not shown)
- CAlpha is the(4 more words not shown)
- DEvery planet in(8 more words not shown)
- EThere is at(6 more words not shown)
Reading the argument
What if there's a planet Beta?
A moon has to orbit some planet. That doesn't tell us which one. The astronaut jumps from that general fact to the specific claim that every moon in S4 orbits Alpha. If S4 contains another planet, some moons might orbit that planet instead.
What we’re looking for
The question asks for an assumption that makes the conclusion follow. My prediction is that Alpha is the only planet in S4. Then every moon in that system would need a planet to orbit, and Alpha would be the only available choice.
Working the answers
- A
One moon could still orbit Beta. Restricting the number of moons doesn't identify the planet that the remaining moon orbits. The conclusion is about where all the moons orbit, not how many moons exist.
- B
The same planet could be Beta. B gathers all the moons around a single planet but never identifies that planet as Alpha. This gets closer to the shape of the conclusion while leaving its crucial name unestablished.
- CCorrect
Alpha is now the only planet in S4. Every moon in the system must orbit a planet, and there is no other planet there for it to orbit. The possibility that motivated our objection has been removed, so all those moons must orbit Alpha.
- D
This allows other planets rather than ruling them out. If both Alpha and Beta existed, each could have several moons, and D would be true. The moons around Beta would make the astronaut's conclusion false. Giving every planet more than one moon doesn't concentrate all moons around Alpha.
- E
At least one isn't all. Alpha could have one moon while Beta had another. E would be true in that situation, but the conclusion that all the system's moons orbit Alpha would still be false. We need to eliminate the other possible destinations, not establish just one Alpha moon.
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