Many corals secrete an exoskeleton that builds up to become a coral reef, and an atoll is the third type of reef. Given these facts, how are atolls built among the following options:
- important habitats
 - By coral exoskeletons
 - by fish
 - with machines
 - by a type of eel
 - It used to be covered by water
 - The area was once underwater
 - reusing materials


 

  By coral exoskeletons       