You are presented with the quiz "How do echinoderms use their feet to locomote themselves?" 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 "echinoderms have a hydraulic system of locomotion called the water vascular system". So, what's the best answer to the question?

OPTIONS:
1). one at a time.
2). to move people.
3). running and lifting.
4). protein channels.
5). a hydraulic system.
6). it needs them.
7). undulations.
8). It helps them survive.
The answer is:
5).