You are presented with the question "Many corals secrete what to form a coral reef?" and the following answer choices: 
- Chlorophyll
 - salt and water.
 - The area was once underwater
 - Chemical energy
 - aqueous solution
 - Organic compounds
 - gastropod shells
 - hard outer covering

Now knowing that many corals secrete an exoskeleton that builds up to become a coral reef and an insect has a hard outer covering called an exoskeleton, choose the best answer.


 

        hard outer covering 