Here's a problem to solve: Which of the following is the primary difference between hydraulic and pneumatic systems?

Among the 4 following options, which is the correct answer?

- A: Hydraulic systems are under pressure and pneumatic systems are not.
 
- B: Hydraulic systems involve liquids and pneumatic systems involve gases.
 
- C: Hydraulic systems are open systems and pneumatic systems are closed systems.
 
- D: Hydraulic systems involve fluids in motion and pneumatic systems involve fluids at rest.
 