Many corals secrete an exoskeleton that builds up to become a coral reef.
An exoskeleton is an external framework.
Many corals secrete an external framework that builds up to become a coral reef.
What do many corals secrete that build up to become a coral reef?
Options:
  * an external framework
  * The area was once underwater
  * that material's properties
  * bioaerosols
  * water
  * dust
  * after too much water received
  * oxygen