You are presented with the quiz "what do coral polyps form?" 

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 "coral reefs are made up of the accumulated exoskeletons of the coral polyp". 

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


 

       coral reef    