Explanation: High-impedance headphones were historically common in radio, particularly with tube-based receivers. Tube output stages, such as a plate circuit, inherently have a high output impedance, often in the kilohm range. High-impedance headphones (e.g., 2000-4000 ohms) present a much closer match to this high output impedance. This "closer match to the plate load impedance" (B) allows for more efficient transfer of the audio signal's power from the tube to the headphones. Consequently, these headphones can often be operated directly or with simple coupling, eliminating the need for a heavy and costly impedance matching transformer (A). This simplified the design and reduced the cost of early radio equipment.