The correct answer is: D. (b) and (c) above
A continuous control actuator is a device that converts an electrical signal into a mechanical motion. It is used to control a process or machine by adjusting the position, speed, or force of a moving part.
Most continuous control actuators use a 4 to 20 mA current loop input. This is a standard way of transmitting signals in industrial control systems. The current loop provides a reliable and accurate way of sending signals over long distances.
Continuous control actuators are also used with standard continuous control loops. A continuous control loop is a feedback system that uses a sensor to measure the output of a process and then adjusts the input to the process to maintain a desired output.
Option A is incorrect because most continuous control actuators are not of the time proportioning type. Time proportioning actuators are a type of actuator that uses a timer to control the output. They are not as common as other types of actuators.
Option B is correct because most continuous control actuators use a 4 to 20 mA current loop input.
Option C is correct because most continuous control actuators are used with standard continuous control loops.
Option E is incorrect because most continuous control actuators are of the type described in options B and C.