Many corals secrete an exoskeleton that builds up to become a coral reef.
Hard corals build by secreting calcium carbonate skeletons.
Corals construct an exoskeleton by secreting calcium  carbonate.
What constructs an exoskeleton using calcium carbonate?
A: animals
B: amoeba
C: sharks
D: insects
E: mammals
F: Man
G: coral
H: bats