Fact 1: Many corals secrete an exoskeleton that builds up to become a coral reef.

Fact 2: An atoll is the third type of reef.

Given the two facts above, answer the question "How are atolls built?" with 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       