Q: How are atolls built?\nOptions:\n- (A) important habitats\n- (B) By coral exoskeletons\n- (C) by fish\n- (D) with machines\n- (E) by a type of eel\n- (F) It used to be covered by water\n- (G) The area was once underwater\n- (H) reusing materials
Let's think step by step below.

User
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.
Thus, the answer is (B).
Assistant