PrepTest 159, Section 3, Question 6
A tiny crab that lives 550 meters below(66 more words not shown)
Which one of the following,(7 more words not shown)
- AA trait that(9 more words not shown)
- BPrey is not(10 more words not shown)
- CThe crabs can(6 more words not shown)
- DIt takes many(7 more words not shown)
- EThe crab's most(4 more words not shown)
Reading the argument
Being able to see something doesn't tell us whether the crab wants to catch it or escape from it.
The crab lives too deep for ultraviolet light from the sun to reach it. So if its sensitivity to that light developed as an adaptation to life down there, light from other organisms is a plausible thing for it to detect. But the argument then picks one particular use: finding prey.
What if the glowing organisms are predators? Seeing something that might eat you could be just as useful as seeing something you might eat. The crab's own lack of bioluminescence doesn't eliminate that possibility.
What we’re looking for
That's the kind of weakener I'd look for: another useful reason to detect bioluminescent organisms. We can accept the crab's ability and the absence of sunlight without accepting the proposed hunting explanation.
Working the answers
- A
This gives a general reason to expect a retained trait to have some use. But the dispute is about which use. Detecting food and avoiding predators could both be functions. A doesn't make the hunting explanation less likely than an alternative.
- B
Scarce prey could make finding prey especially important. At any rate, saying food is hard to find doesn't tell us that ultraviolet sensitivity serves some other purpose. We still don't know what the crab uses the ability to detect.
- C
Detecting other crabs without vision doesn't establish why the crab sees ultraviolet light. It also doesn't mean the crab never uses vision; having one method of detection doesn't rule out another. This supplies no reason to reject hunting in favor of a different use for ultraviolet sensitivity.
- D
The time needed to develop the ability doesn't identify its function. It could take many generations to develop an ability used for hunting or one used for avoiding danger.
- ECorrect
Now the predator possibility has support: the crab's most dangerous predators emit light. Detecting that light could help the crab avoid being eaten. We don't have to prove that it never hunts bioluminescent prey. The answer weakens the argument by giving a serious alternative explanation for the ability the argument attributes to hunting.
More Weaken questions
Learn the LSAT our way
Our platform drills Weaken questions at your level and adjusts as you improve.