PrepTest 143, Section 3, Question 25
The populations of certain species of amphibians have(51 more words not shown)
The argument depends on assuming(5 more words not shown)
- AThe amphibian species(35 more words not shown)
- BThe variations in(28 more words not shown)
- CEither industrial pollution(21 more words not shown)
- DIf industrial pollution(25 more words not shown)
- EIf industrial pollution(14 more words not shown)
Reading the argument
A population can fluctuate and still have a downward trend.
I can accept that weather makes amphibian populations rise and fall while still wondering whether pollution helps explain their recent decline. Normal variation doesn't automatically erase another influence. A series of ups and downs could occur around a trend that keeps moving lower.
The author's “impossible to be sure” claim also makes me pause. We could investigate whether exposure to pollution harms the animals, for example. That wouldn't automatically identify the cause of these particular historical declines, but it shows why we should ask what evidence is available before treating ordinary fluctuations as the end of the inquiry.
What we’re looking for
The question asks for an assumption the argument depends on. We need to examine what the author must be taking for granted when using the general weather pattern to cast doubt on the scientists' explanation.
Working the answers
- ACorrect
Start with the species whose declines scientists attribute to pollution. Then identify the other group in the answer: species whose populations do not vary greatly with natural weather changes. Those are exceptions to the broad pattern in the passage. A says the affected species are not known to be among those exceptions.
Why does that matter? Suppose we did know these particular species were exceptions: their populations don't fluctuate greatly with the weather. Saying that most other amphibian populations do fluctuate would then give us no stated reason to explain away the declines in these species. The author's weather-based objection needs that escape route to remain open.
Notice the careful wording. A doesn't establish that every affected species definitely fluctuates with weather. It says we don't know that these species are exceptions to the weather pattern. And denying A wouldn't prove pollution caused the declines; it would remove the particular reason the author gives for insisting that we can't be sure.
- B
The argument doesn't need natural variations to be smaller than some of the declines. If natural weather changes could produce equally large or larger variations, the author's doubt about attributing a decline to pollution could still make sense. B's particular size comparison isn't required.
- C
Both factors could contribute. Pollution could lower a population while weather changes also push it up or down. The argument can question whether pollution explains the observed decline without assuming that only one cause is allowed.
- D
The damage could persist even after pollution is reduced. Nothing in the argument requires population recovery to follow a reduction, or a further population drop to track every increase in pollution. The argument concerns what explains recent observations, not a guaranteed response to future changes.
- E
The author doesn't need pollution to change the weather. Natural weather variation could provide a competing explanation without being caused by pollution at all. Whether severe pollution can create additional weather variation is a different causal connection from the one the argument relies on.
More Necessary Assumption questions
Learn the LSAT our way
Our platform drills Necessary Assumption questions at your level and adjusts as you improve.