PrepTest 143, Section 4, Question 21
In modern deep-diving marine mammals, such as whales,(63 more words not shown)
Which one of the following,(6 more words not shown)
- ASome deep-diving marine(14 more words not shown)
- BIn most modern(12 more words not shown)
- CIn most modern(18 more words not shown)
- DIn addition to(30 more words not shown)
- EThere is evidence(21 more words not shown)
Reading the argument
Maybe those bones were useful for something other than deep diving.
I can see how light bones would help a whale return to the surface. But finding the same porous outer shell in an ichthyosaur doesn't tell us that the reptile used it for the same activity. A feature that helps an animal do something isn't proof that the animal did it.
What we’re looking for
To weaken the inference, I'd like an example of porous bones without deep diving. That would show why the bones alone don't establish the behavior. We don't need to deny that ichthyosaurs could have been deep divers. We need to show that this particular evidence leaves another possibility open.
Working the answers
- A
This gives us deep divers without porous bones. That shows the bones aren't required for deep diving, but the author's inference runs the other way: these animals had porous bones, so they must have dived deeply. We want porous bones without the behavior, not the behavior without porous bones.
- B
Most modern marine reptiles lacking porous bones tells us little about what follows when a reptile does have them. B also doesn't tell us whether those modern species dive deeply. It supplies neither side of the counterexample we're looking for.
- CCorrect
Most non-deep-diving marine reptile species, modern and prehistoric, have porous outer bone shells. That directly shows the feature can exist without deep diving. The ichthyosaur's porous bones could therefore fit a reptile that wasn't a deep diver, weakening the attempt to infer its behavior from those bones alone.
C doesn't prove ichthyosaurs stayed near the surface. It removes the confidence the author places in this feature as evidence of deep diving.
- D
Whales have other features suited to deep diving, but we don't know whether ichthyosaurs shared them. Unknown isn't the same as absent. D also doesn't establish that those additional features are required for deep diving. C is more direct: it gives animals with the supposedly revealing bone structure that do not dive deeply.
- E
The bones could have been light enough without porous outer shells. That means porosity wasn't the only possible way to make them light; it doesn't tell us whether the ichthyosaurs actually dived deeply. An animal could have more than one feature helping it surface and still be a deep diver. E doesn't supply the missing counterexample.
More Weaken questions
Learn the LSAT our way
Our platform drills Weaken questions at your level and adjusts as you improve.