You are presented with the quiz "what have a unique water vascular system with tube feet?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "echinoderms have a unique water vascular system with tube feet" and "starfish and sea urchins are also echinoderms". 

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


 

   sea urchins        