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? ['Until they are fully grown, horseshoe crabs shed their shells and grow new ones several times a year.', 'The strength of the tidal currents in a given location varies widely over the course of a day.', 'A very large barnacle population can significantly decrease the ability of a horseshoe crab to find food.', 'Tidal currents do not themselves dislodge barnacles from the shells of horseshoe crabs.']