You are presented with the question "A reef is made of" and the following answer choices: 
- it keeps an organism warm
 - salt and water.
 - granite
 - magnesium
 - gastropod shells
 - ocean water
 - calcium carbonate
 - zinc

Now knowing that many corals secrete an exoskeleton that builds up to become a coral reef and coral is calcium carbonate with a trade of carotene, choose the best answer.


 

       calcium carbonate  