PrepTest 143

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Phenomenon Ð Some amphibian populations have declined dramatically
Scientists's explanation Ð It's caused by industrial pollution
Rebuttal Ð Most amphibian species' populations fluctuate greatly based on natural weather variations
Conclusion Ð We can't be sure industrial pollution is the cause

Answer Anticipation:
This argument falls into a common structure we've seen time and again on the LSAT—phenomenon/explanation. However, in this case, the author brings up an explanation suggested by an opposing point, concluding that we can't be certain that's the real explanation.

How does the author undermine the view of the scientists? She brings up an alternative possible explanation—it's not industrial pollution, but rather natural variations in the weather driving normal population changes.

A viable alternative explanation means you can't be certain of a given explanation. Since we know there's an assumption in this argument (based on the question), then, it must be the case that this alternative isn't established as being a viable alternative. In other words, it's viability must be based on an assumption.

Looking at the language of the explanation, one word jumps out—"most." That doesn't cover all amphibian species, and the phenomenon was about "certain" species. There's no indication that the species with the natural, weather-based population fluctuations are the ones that are seeing the dramatic decrease in recent years. If they are, then the alternative explanation is viable. If not, then this alternative isn't a viable counter to the scientists's theory. The argument needs these certain species of amphibians to be in the majority group that's affected by weather changes.

Answer Explanation:
Wow, way to try to hide the right answer behind a double negative, LSAT! Despite the confusing language, this answer choice raises the issue we identified in our anticipation, so you should have dug in deeper. Crossing out the double negative, this answer states that the species whose declines are the subject of the scientists's theory are known to be among those whose populations vary with the weather. If they're members of those species, then the alternative explanation is viable and it serves as an effective counter to the scientists. If they're not, then the alternative theory isn't viable and the argument falls apart.

Key Takeaway:
There were a lot of distractions in this question, but at the end of the day it ended up being about overlapping sets—whether "certain species" were included in "most species" that had a characteristic. This should highlight that these quantifier words are important on the exam, and you should be noting them as they come up and tracking them to see if there are any shifts that highlight a flaw in the argument.

Incorrect. The closer the alternative theory matches the data, the better supported it is. If, as this answer states, weather fluctuations drive population change that isn't always as big as what's being seen, then the argument is actually weakened.

Incorrect. Since the conclusion of the argument is that it's impossible to be sure industrial pollution is the cause, there being other viable alternative theories would only serve to strengthen the argument, so there's no need to assume that these are the only two possibilities.

Incorrect. This answer supports the view of the scientists, not the argument, so it's incorrect.

Incorrect. This answer bridges the gap between the two proposed explanations, which isn't necessary. Additionally, severe industrial pollution is out of scope.