You are presented with the quiz "fish, amphibians, reptiles, birds, and mammals have what?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "vertebrate chordates have a backbone" and "chordates Chordates Chordates include fish, amphibians, reptiles, birds, and mammals". 

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


 

   backbone        