Which component is listed as present in hydraulic system D but not in hydraulic system C?

• A. Check valves
• B. Accumulator
• C. Pressure gauge
• D. Relief valve

Answer: (A)

Check valves are used to allow fluid to flow in only one direction, preventing backflow in parts of a hydraulic circuit where pressure can drive fluid the wrong way when a pump stops or a valve shifts. If hydraulic system D includes a check valve while system C does not, that difference signals a design where backflow must be blocked in D to keep a component or line from draining or coasting back, or to maintain a held position in the off state. The other components—an accumulator, pressure gauge, and relief valve—serve energy storage, monitoring, and overpressure protection, respectively, and their presence alone doesn’t explain why one diagram would require backflow prevention while the other doesn’t. So the inclusion of a check valve in system D points to a need to stop reverse flow in that path, which is not indicated for system C.