What are shells made of? what happens when they fall to the bottom of he ocean?

Most shells that are found on beaches are the shells of marine mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin. Apart from mollusk shells, other shells that can be found on beaches are those of barnacles, horseshoe crabs and brachiopods.

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.