Which statement correctly describes hydraulics versus pneumatics?

• A. Hydraulics uses oil and pneumatics uses gas
• B. Hydraulics uses gas and pneumatics uses oil
• C. Both use oil
• D. Both use gas

Answer: (A)

Hydraulics vs pneumatics mainly differs in the working fluid: hydraulics use a liquid (typically oil) and pneumatics use a gas (air). This fuels the distinct behaviors: oil is nearly incompressible, so pressure changes transmit force very efficiently and you can generate large, controllable forces with compact actuators. Air, being compressible, stores energy and moves quickly, which makes pneumatic systems fast and lightweight but generally capable of lower and more variable forces. So the statement that hydraulics uses oil and pneumatics uses gas captures the fundamental distinction. The other options conflict with these fluid properties: hydraulics does not rely on gas, pneumatics does not rely on oil, and they do not both use the same fluid.