Critical Reasoning

In order to withstand tidal currents, juvenile horseshoe crabs frequently burrow in the sand. Such burrowing discourages barnacles from clinging to their shells. When fully grown, however, the crabs can readily withstand tidal currents without burrowing, and thus they acquire substantial populations of barnacles. Surprisingly, in areas where tidal currents are very weak, juvenile horseshoe crabs are found not to have significant barnacle populations, even though they seldom burrow.

Which of the following, if true, most helps to explain the surprising finding?

(A) Tidal currents do not themselves dislodge barnacles from the shells of horseshoe crabs.
(B) Barnacles most readily attach themselves to horseshoe crabs in areas where tidal currents are weakest.
(C) The strength of the tidal currents in a given location varies widely over the course of a day.
(D) A very large barnacle population can significantly decrease the ability of a horseshoe crab to find food.
(E) Until they are fully grown, horseshoe crabs shed their shells and grow new ones several times a year.

OA:E

Source-OG 17

This is a really hard question for non-native speakers of English. I can't tell you how many times a student has come to me asking, "Okay. What are tidal currents, what are horseshoes, what are crabs, what is burrow, and what are barnacles?"

Then, when the student finds out that (E) is the best answer, I get asked, "What does shed mean?"

This question is a resolve/explain question. We have two seemingly contradictory premises:

A) Juvenile horseshoe crabs do not burrow where tidal currents are weak.
B) Juvenile horseshoe crabs do not have significant barnacle populations.

Since barnacles readily attach to crabs that don't burrow, this is surprising. How can this be explained?

A) is irrelevant and, even if it were true, it would not explain the discrepancy.
B) does not explain the discrepancy. If anything it worsens our puzzlement.
C) is irrelevant information.
D) is more irrelevant information.

E) explains the discrepancy. Since horseshoe crabs shed (throw off) their shells on a regular basis, the barnacles will be attached to the old shell while the juveniles grow a new one.