Many corals secrete an exoskeleton that builds up to become a coral reef, and an exoskeleton is an external framework. Given these facts, what do many corals secrete that build up to become a coral reef among the following options:
- an external framework
 - The area was once underwater
 - that material's properties
 - bioaerosols
 - water
 - dust
 - after too much water received
 - oxygen


 

 an external framework        