Many corals secrete an exoskeleton that builds up to become a coral reef.
Corals, and invertebrates in general, are delicate creatures.
Some invertebrates secrete an exoskeleton that builds up to become a reef.
What can secrete an exoskeleton that builds up to become a reef?
Options:
A: graptolites
B: some invertebrates
C: remains of prehistoric life
D: humans
E: insects
F: gastropod shells
G: barnacles
H: all invertebrates