PrepTest 139

Incorrect. This answer is too extreme to be necessary. If there are a few locations where there's a thick leaf layer without goblin ferns, the argument could still hold together. The problem would be if there are goblin ferns in areas with a lot of earthworms, as they're stated to be the cause of the disappearance but they're not tied to the leaf layer.

Incorrect. The argument is about a single type of earthworm, so an answer about all types of earthworm is too broad.

Incorrect. If the argument concluded that the thin leaf layer was responsible for the disappearance of ferns, then this answer might be relevant (and would weaken the argument by suggesting reversed causality).

Incorrect. This answer is too extreme. If the earthworms are the cause of the disappearance, presumably there's some overlap after the earthworms move into an area but before the ferns disappear. As such, if anything, this answer might actually call into question the argument by stating that the cause isn't present where the effect is (there are no areas with disappearing ferns that also have the earthworms).

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Requirement - Goblin ferns require a thick leaf layer
Situation - These ferns are disappearing. Where that happens, the leaf layer is thin and there are worms; where it doesn't, there are no worms
Conclusion - The worms are probably the cause of the disappearance

Answer Anticipation:
Causal language in the conclusion ("responsible for")? Time to check for a correlation/causation flaw.

The premises describe the situation facing the goblin fern, and it does so by bringing up a correlation. Where the fern is disappearing, the leaf layer is thin and there are earthworms. Where it isn't, there are no earthworms. That correlation is used to justify the conclusion that the earthworms are probably the cause of the disappearing fern.

Since this argument commits a correlation/causation flaw, we should look for answers that address the common reasons that this jump is flawed:
(1) A third cause that explains the correlated phenomena
(2) A coincidence (rarely correct on the LSAT)
(3) Reversed causality

And since this is a Strengthen with Necessary Premise question, we're looking for an answer that rules out one of these alternative explanations for the correlation.

Answer Explanation:
This answer rules out a possibility that would undercut the argument. It's possible that something else thins out the leaf layer, thus causing the disappearance of the goblin fern while also attracting the earthworms. To match this with common LSAT patterns, this answer suggests that there isn't a third cause that results in the thinning of the leaf layer and thus the disappearance, and separately the earthworms being present. In ruling out an alternative explanation, this answer is necessary to conclude that the given explanation is probable.

Key Takeaway:
A correlation/causation jump is flawed for a few set reasons. Knowing how each of those reasons can show up in a correct answer choice in different question types is important to answering these questions correctly. For instance, in a Strengthen question (any variety), ruling out alternative possible causes is one potentially correct answer; in a Weaken question, identifying an alternative possible cause is the name of the game.