Fact 1: Echinoderms have a unique water vascular system with tube feet.

Fact 2: Sea stars use hundreds of tube feet.

Given the two facts above, answer the question "Sea stars use a unique water vascular system with what?" with the following options: 
- growth
 - a ray
 - feet
 - oxygen
 - porous
 - energy
 - Light
 - volume


 

   feet      