You are presented with the quiz "What do many corals secrete that build up to become a coral reef?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "many corals secrete an exoskeleton that builds up to become a coral reef" and "an exoskeleton is an external framework". 

So, what's the best answer to the question?


 

 an external framework          