The goblin fern, which requires a thick layer of leaf litter on the fo

OA -> E
Opposite is not true i.e L rubellus is not there because leaf layer is already thin.

The trigger "˜thus’ helps us locate the conclusion here: L. rubellus is probably responsible for the fern’s disappearance. This is a causation conclusion: L is causing the fern to disappear. What is this conclusion based on? areas where fern recently vanished are teeming with L and have unusually thin leaf litter (and we’re told L eats leaf litter). It certainly seems reasonable to conclude that this is all L’s fault, but we know we must be very careful when making a causation conclusion. The premises offer us a correlation, but that’s never enough to prove causation. Perhaps an unknown factor is causing the fern to vanish, and L didn’t enter those areas until later? This could all just be one big misunderstanding! Let’s diagram the core:

Spots where fern recently vanished have unusually thin leaf litter, which is required by the fern + those spots are teeming with L, which eats leaf litter --> L is probably responsible for the fern’s disappearance

We’re looking for an answer choice that will make a coincidence less likely, and we’ll get rid of answer choices if negating them doesn’t kill the conclusion:

(A) doesn’t mention L so probably wrong; let’s negate it and see what happens: some North American forests with thick leaf litter don’t have any fern. No one ever said fern has to be in every spot that has thick leaf litter! So, even if the opposite of (A) is true, L could still be the reason for the fern’s disappearance. Get rid of this one.

(B) Let’s negate this one too: some earthworms other than L eat leaf litter. Even so, L could still be responsible for the fern’s disappearance. Get rid of this one.

(C) You’d probably expect to find evidence of dead fern in areas where the fern recently vanished. What if the fern’s dead leaves only made up the smaller part of the leaf litter (again, negation)? This changes nothing... L could still be responsible for the fern’s disappearance. Get rid of this one.

(D) Try negation again: what if you could find some spots in North American forests that have both L and fern? Even then, L could still be responsible for the fern’s disappearance _ perhaps the fern will disappear from those spots as well in the near future. Get rid of this one.

(E) Negation has worked really well for us in the first four answer choices, and at this point we’re hoping (E) is right, otherwise we’re in trouble. Let’s try negating this one as well: L favors habitats where the leaf litter layer is considerably thinner than what is required by the fern. Aha! This means that the fern vanished before L arrived! L couldn’t be responsible for the fern’s disappearance!

So (E) is correct.