Given the question: 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.
The answer is:
calcium carbonate