Explanation: Adjusting a transmitter's power output involves both regulatory compliance and practical circuit design. Firstly, amateur radio regulations (FCC Part 97.313(a)) explicitly state that an amateur station must use the minimum transmitting power necessary to accomplish the desired communication. This minimizes interference to other users and ensures efficient operation, making option A a correct regulatory principle. Secondly, from a radio theory perspective, the power output of many transmitters, especially those using vacuum tubes or even solid-state devices, can be controlled by varying the supply voltage to the final amplifier stage. For vacuum tube amplifiers, increasing or decreasing the plate supply voltage directly impacts the power output, as power is proportional to voltage times current. This method is a common way to adjust power, making option B a correct statement of practical circuit control. Since both the regulatory requirement for minimum power (A) and the technical method of varying supply voltage (B) are valid aspects of adjusting transmitter power, option C, combining both, is the most comprehensive answer.