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

Fact 2: Coral is calcium carbonate with a trade of carotene.

Given the two facts above, answer the question "A reef is made of" with the following options: 
- it keeps an organism warm
 - salt and water.
 - granite
 - magnesium
 - gastropod shells
 - ocean water
 - calcium carbonate
 - zinc


 

       calcium carbonate  