Many corals secrete an exoskeleton that builds up to become a coral reef, and an insect has a hard outer covering called an exoskeleton. Given these facts, many corals secrete what to form a coral reef among 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 