Final answer:
A control system controls the output based on the input, and the correct answer to the fill-in-the-blank question is 'output' and 'input' respectively.
Step-by-step explanation:
A control system is a combination of devices connected together in such a way that the magnitude of some physical quantity called the output is controlled by the magnitude of some other physical quantity called the input. These two quantities may not have the same units. Hence the answer to the student's question is A) Input; output.
The control systems often use a sensor to monitor the value of the variable (output) and send this data to a control center. If the output deviates from the desired value, or setpoint, the control center generates an error signal. The control system responds to this error to adjust the output back to the setpoint or normal range, typically using an actuator. This process is known as negative feedback.