You are presented with the quiz "Coral reefs are made up of the exoskeletons of thousands of what?" 

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 is made of the exoskeletons of many thousands of tiny marine animals called polyps". 

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


 

    tiny polyps       