PrepTest 158, Section 3, Question 20
The study of primates is interesting for many(66 more words not shown)
Which one of the following(8 more words not shown)
- AThe chimpanzee, a(6 more words not shown)
- BNo primates indigenous(6 more words not shown)
- CNo higher primate(2 more words not shown)
- DThere are some(4 more words not shown)
- EThere are no(2 more words not shown)
Reading the argument
There are several facts here, so keep them separate before trying to combine them.
Only primates have opposable thumbs. That makes being a primate a requirement for having those thumbs; it doesn't tell us that every primate has them. Lemurs are lower primates, and they are the only primates indigenous to Madagascar. Some lemur species are also the only living lower primates active mainly during the day. Finally, higher primates are thought to descend from a diurnal lower-primate species.
What we’re looking for
I don't have one precise prediction to force onto the answers. The inference question asks what these facts establish, so let's test each choice against the particular information it needs. Especially watch for a shift from living species to an ancient ancestor, or from some lemurs to all lemurs.
Working the answers
- A
The proposed ancestor was a diurnal lower primate. The passage identifies some lemurs as the only living lower primates of that kind. The word “living” matters: the ancestral species could have gone extinct.
Imagine an earlier species from which both lemurs and higher primates descended. Then the ancestor wouldn't have to be a lemur, even though only some lemurs fill the description among living lower primates. A gives an identity the passage doesn't establish.
- BCorrect
Any primate indigenous to Madagascar must be a lemur, and lemurs are lower primates. So there are no higher primates indigenous to Madagascar. That rules out diurnal higher primates in particular.
We don't need the information about thumbs or ancestry to reach this answer. The two facts about lemurs and Madagascar do all the work. Having extra information in the passage doesn't mean every fact must appear in the inference.
- C
Being descended from a diurnal species doesn't establish that every present-day higher primate is diurnal. The passage gives the ancestor's activity pattern, not a rule that descendants must preserve it. We can't rule out nocturnal higher primates on this information.
- D
Some lemurs could lack opposable thumbs, but we haven't established that any actually do. The opening rule only says that an animal with opposable thumbs must be a primate. It leaves open whether all primates, including all lemurs, have them. It's one thing to say they could lack thumbs; it's another to say some do.
- E
Some lemur species are diurnal. That doesn't settle the activity patterns of the remaining lemur species, so it doesn't rule out nocturnal lemurs. We don't need to prove that nocturnal lemurs exist to reject E; we only need to see that their absence hasn't been established.
More Must Be True questions
Learn the LSAT our way
Our platform drills Must Be True questions at your level and adjusts as you improve.