Many corals secrete an exoskeleton that builds up to become a coral reef, and coral is calcium carbonate with a trade of carotene. Given these facts, a reef is made of among the following options:
- it keeps an organism warm
 - salt and water.
 - granite
 - magnesium
 - gastropod shells
 - ocean water
 - calcium carbonate
 - zinc


 

       calcium carbonate  