You are presented with the quiz "What is secreted by organisms like frogspawn 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 "good examples are Bubble coral, Elegance coral, Frogspawn and other Euphyllia corals". 

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


 

   exoskeleton        