You are presented with the quiz "What can secrete an exoskeleton that builds up to become a 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 "corals, and invertebrates in general, are delicate creatures". 

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


 

  some invertebrates         