PrepTest 113, Section 2, Question 26
All known deposits of the mineral tanzanite are(21 more words not shown)
Which one of the following(10 more words not shown)
- AThe lagoon on(33 more words not shown)
- BEvery frog ever(36 more words not shown)
- CFrogs are the(42 more words not shown)
- DThe only frogs(39 more words not shown)
- EEach frog on(37 more words not shown)
Reading the argument
Where else would she get one?
Ashley collects only tanzanite, and every deposit we know about is in Tanzania. That makes it reasonable to expect her stones to originate there. Notice the limits, though: these are the known deposits, and the conclusion says she is unlikely to collect a stone from somewhere else. It leaves room for an undiscovered source. We don't need to attack a cautious prediction as though the author claimed absolute certainty.
What we’re looking for
For a parallel, I want three roles: everything of a certain kind that we've discovered comes from one place; someone restricts their collection to that kind; so they probably won't collect something from outside that place. The collection could become a diet or some other activity. What matters is preserving who is restricted to what—and preserving the conclusion's degree of confidence.
Working the answers
- A
Many frogs in the lagoon isn't enough. There could also be plenty of frogs in a creek or elsewhere on the island, and the owls could eat those. We need the lagoon to contain every known island frog, just as Tanzania contains every known tanzanite deposit. A doesn't establish that starting restriction.
- B
The first sentence looks right: every frog we've seen is in the lagoon. But then the restriction points the wrong way. Saying frogs are eaten only by owls limits which animals eat the frogs. It doesn't limit what the owls eat. They could also eat animals from outside the lagoon.
To match Ashley's collection, we'd need the owls to eat only island frogs. Ashley collects only tanzanite; we weren't merely told that only Ashley collects tanzanite.
- C
This tells us what kinds of animals are known to live in the lagoon. We need to know where the frogs live. There could be frogs elsewhere on the island even if the lagoon contains nothing but frogs.
Compare the mineral version: “Tanzanite is the only mineral known in Tanzania” wouldn't mean that all known tanzanite comes from Tanzania. C changes that first relationship.
- DCorrect
Match the roles: the island frogs are the tanzanite, the lagoon is Tanzania, and the owls' diet is Ashley's collection. All discovered island frogs live in the lagoon, and the owls eat only island frogs. So the owls probably won't eat an animal from outside the lagoon.
The uncertainty is in the same place too. We know where the discovered frogs live, but an undiscovered population could exist elsewhere. That fits a probable conclusion, just as the possibility of an unknown tanzanite deposit fits the original's “unlikely.”
- E
This is close, so compare both the premises and the conclusion. E states without qualification that every island frog lives in the lagoon. It puts the uncertainty in the owls' diet instead: they aren't known to eat anything else. Then it draws an absolute conclusion that they will never eat anything from outside the lagoon.
The original does the opposite with those qualifications. The location claim is limited to known deposits, Ashley's collection is definitely restricted to tanzanite, and the conclusion remains a prediction. E moves the uncertainty and then claims more certainty than the evidence provides.
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