You are presented with the question "What do many corals secrete that build up to become a coral reef?" and the following answer choices: 
- an external framework
 - The area was once underwater
 - that material's properties
 - bioaerosols
 - water
 - dust
 - after too much water received
 - oxygen

Now knowing that many corals secrete an exoskeleton that builds up to become a coral reef and an exoskeleton is an external framework, choose the best answer.


 

 an external framework        