Many corals secrete an exoskeleton that builds up to become a coral reef.
An atoll is the third type of reef.
Atolls are built by corals secreting an exoskeleton.
How are atolls built?
Options:
(A) important habitats
(B) By coral exoskeletons
(C) by fish
(D) with machines
(E) by a type of eel
(F) It used to be covered by water
(G) The area was once underwater
(H) reusing materials