Answer the following question given this paragraph: 

A unique feature of echinoderms is their water vascular system . This is a network of canals that extend along each body part. In most echinoderms, the canals have external projections called tube feet (see Figure below ). The feet have suckers on the ends. Muscle contractions force water into the feet, causing them to extend outward. As the feet extend, they attach their suckers to new locations, farther away from their previous points of attachment. This results in a slow but powerful form of movement. The suckers are very strong. They can even be used to pry open the shells of prey.


Q: What makes echinoderms force water into the feet and move forward?

 Choices:

- spring contractions

- vacuum contractions

- field contractions

- muscle contractions

A:
muscle contractions
