You are presented with the quiz "The two-chambered heart of some backboned aquatic animals is part of their:" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "fish have a circulatory system with a two-chambered heart" and "fish are backboned aquatic animals". 

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


 

      circulatory system     