Central direct digital control systems are typically used for:

• A. Standalone local control of equipment.
• B. Providing only boundary temperature sensing.
• C. Supervisory control instead of closed loop control.
• D. Real-time direct actuation at field devices.

Answer: (C)

Central direct digital control systems are about supervisory control over a network of field devices. In this setup, sensors and actuators at the equipment are typically managed by local DDC controllers that handle the fast, real-time closed-loop control. The central system collects data, provides monitoring and alarms, logs trends, and issues setpoints or overrides as needed. So the central DDC acts to supervise and coordinate the overall process rather than directly driving each control loop in real time at the field devices. The other options don’t fit because standalone local control is handled at the field level, boundary sensing is just data collection, and real-time direct actuation would be performed by the local controllers, not the central supervisory layer.