Acadia is beautiful even in the rain and fog, but the park still doesn't have many sandy beaches, and this is surely not a sandy beach, the rocks are granite, broken off the granite bedrock. Why aren't there sandy beaches?

The sand is supplied to the beach more slowly than it is lost to the deep waters. This prevents sand beaches from forming.

Explanation:

The formation of sandy beaches depends on a set of environmental forces that must interact with each other. These forces are summed up by the speed at which the sand is added to the solid part of the beach and the speed at which the sand is taken by the deep waters. This justifies the reason why sandy beaches are not formed in Acadia, because the speed at which the sand is captured by the deep waters, is greater than the speed at which the sand is added to the solid part of the beach.

a is the answer i believe.