In process control, most continuous control actuators are designed to accept standard analog control signals (commonly a 4–20 mA current loop) and are used within continuous control loops; many are not merely time-proportioning devices.

Introduction / Context:
Continuous control actuators—such as control valves with positioners, variable-speed drives, and servo actuators—receive analog commands representing desired setpoints. The most common industrial standard is a 4–20 mA current loop, chosen for noise immunity and simple fault detection (0 mA implies a break). These actuators operate inside continuous feedback loops (e.g., PID) to maintain process variables at setpoints.

Given Data / Assumptions:

• We are discussing continuous (analog) control rather than on/off control.
• Actuators accept proportional commands corresponding to physical positions or speeds.
• Standard process control signals are in use.

Concept / Approach:
In a continuous loop, a controller computes an analog output (e.g., 4–20 mA) based on error = setpoint - measured value. The actuator responds proportionally, enabling fine control. Time-proportioning (duty-cycle) schemes are typical for discrete actuators (e.g., heaters with relays) rather than for continuous actuators, so labeling “most continuous actuators” as time-proportioning would be misleading.

Step-by-Step Solution:
Identify the industry-standard signal: 4–20 mA input.Recognize use within continuous control loops (PID, cascade).Reject the generalization that most continuous actuators are time-proportioning.Select the combined answer (b) and (c).

Verification / Alternative check:
Valve positioners and VFDs list 4–20 mA as standard control input; loop tuning procedures assume continuous proportional response, confirming (b) and (c).

Why Other Options Are Wrong:
Time proportioning: more aligned with on/off control, not most continuous actuators.(b) only or (c) only: each is true but incomplete.None of the above: incorrect because (b) and (c) are correct.

Common Pitfalls:
Confusing PWM/duty-cycle methods with analog positioning; ignoring fail-safe current ranges and signal scaling; neglecting loop integrity checks.

Final Answer:
(b) and (c) above