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

Fact 2: An insect has a hard outer covering called an exoskeleton.

Given the two facts above, answer the question "Many corals secrete what to form a coral reef?" with the following options: 
- Chlorophyll
 - salt and water.
 - The area was once underwater
 - Chemical energy
 - aqueous solution
 - Organic compounds
 - gastropod shells
 - hard outer covering


 

        hard outer covering 