Hydraulic systems are under pressure and pneumatic systems are not. Hydraulic systems involve liquids and pneumatic systems involve gases. Hydraulic systems are open systems and pneumatic systems are closed systems. Hydraulic systems involve fluids in motion and pneumatic systems involve fluids at rest.
I gave my students this multiple choice question: Which of the following is the primary difference between hydraulic and pneumatic systems?

Only one answer is correct among these 4 choices:
- Hydraulic systems are under pressure and pneumatic systems are not.
- Hydraulic systems involve liquids and pneumatic systems involve gases.
- Hydraulic systems are open systems and pneumatic systems are closed systems.
- Hydraulic systems involve fluids in motion and pneumatic systems involve fluids at rest.

Could you tell me which one is correct?

Hydraulic systems involve liquids and pneumatic systems involve gases.