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 signi� cant barnacle
populations, even though they seldom burrow.
Which of the following, if true, most helps to explain the surprising � nding?
(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 signi� cantly decrease the ability of a horseshoe crab to � nd food.
(E) Until they are fully grown, horseshoe crabs shed their shells and grow new ones several times a year.

A says that tidal currents do not by themselves dislodge the barnacles. But this is stated in the argument also. Refer the line "Such burrowing discourages barnacles from clinging to their shells" which implies that the burrowing dislodge the barnacles.

B goes against the argument and does nothing to help explain the surprise finding.

C is not being discussed here and daily variations in tidal strength is not the concern.

D may explain the argument but it is incorrect because the passage states that " When fully grown, however, the crabs can readily withstand tidal currents without burrowing, and thus they acquire substantial populations of barnacles". If D was correct then even full grown crabs would not have collected the barnacles.

E correctly explain the anomaly because if the young crabs shed their shells and grow new ones several times a year, then it shows that the low population of barnacles has little to do with the tidal strengths and explains why barnacles are not present in significant amounts in both high tide and low tide areas.

Hope this clears your doubt!

Birottam Dutta wrote: D may explain the argument but it is incorrect because the passage states that " When fully grown, however, the crabs can readily withstand tidal currents without burrowing, and thus they acquire substantial populations of barnacles". If D was correct then even full grown crabs would not have collected the barnacles.

Hope this clears your doubt!

I'm still not clear with this option. D option even states the ability of a horseshoe to find food ?
This is nowhere mentioned in the passage and therefore would be incorrect isn't it ?

Some corrections to the info posted which doesn't really answer your question. Horseshoe Crabs, after the first couple years of life,only molt once a year on average. Also, they secrete a mucous substance that coats the outer carapace. This substance is thought to prevent non-mobile organisms from setting up housekeeping on the horseshoe crabs. As the crabs age, they make less and less of the mucous, and therefore have more animals living on them.