The Following Questions Refer to the Paragraph Below

The following questions refer to the paragraph below.
An elementary school science teacher decided to liven up the classroom with a salt-water aquarium. Knowing that salt-water aquaria can be quite a hassle, the teacher proceeded stepwise. First, the teacher conditioned the water. Next, the teacher decided to stock the tank with various marine invertebrates, including a polychaete, a siliceous sponge, several bivalves, a shrimp, several sea anemones of different types, a colonial hydra, a few coral species, an ectoproct, a sea star, and several gastropod varieties. Lastly, some vertebrates-a parrotfish and a clownfish-were added. She arranged for daily feedings of copepods and feeder fish.
-Parrotfish have mouths adapted to scrape algae off of coral, and can even munch on coral. The aquarium's corals rapidly dwindled; in their place were shards of

A) chitin.
B) calcium carbonate.
C) silica.
D) bone.
E) chitin impregnated with calcium carbonate.