You are presented with the quiz "Many corals secrete what to form 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 insect has a hard outer covering called an exoskeleton". 

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


 

        hard outer covering   