You are presented with the quiz "What constructs an exoskeleton using calcium carbonate?" 

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 "hard corals build by secreting calcium carbonate skeletons". 

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


 

       coral    