You are presented with the question "How are atolls built?" and the following answer choices: 
- 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

Now knowing that many corals secrete an exoskeleton that builds up to become a coral reef and an atoll is the third type of reef, choose the best answer.


 

  By coral exoskeletons       