Explanation: Radiotelegraph keying involves interrupting current flow at a suitable point in the transmitter to create the Morse code signal. **Option A** presents several valid, albeit some older, methods: * **Primary of power transformer:** While not ideal due to slow rise/fall times and potential for chirp, keying the AC primary of the power transformer would effectively turn the entire transmitter on and off, thus accomplishing keying. * **Filament centertap:** This is a form of cathode keying. By opening the cathode circuit (often via the filament transformer centertap for indirectly heated cathodes), the tube's current flow is interrupted. * **Screen grid circuit:** Keying the screen grid voltage is a common and effective method, known as screen grid keying. Modulating the screen voltage controls the tube's ability to conduct. **Option B** also lists common keying points: * **Control grid circuit:** "Grid-block keying" is a widely used and effective method. By applying a sufficiently negative voltage to the control grid during key-up, the tube is biased into cutoff, preventing current flow. * **DC supply:** Keying the DC high voltage (B+) supply to a stage (e.g., the final amplifier) is a form of "plate keying." This directly interrupts the power to the amplifying stage. Since all points listed in both A and B are locations where keying can be accomplished, option C (A or B) is the correct choice.