Fact 1: Many corals secrete an exoskeleton that builds up to become a coral reef.

Fact 2: An exoskeleton is an external framework.

Given the two facts above, answer the question "What do many corals secrete that build up to become a coral reef?" with 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        