True about the two-wire circuit is:

• A. They do not require electrical power to operate
• B. They require a battery
• C. They rely on central power
• D. They are fragile

Answer: (A)

The key point is that a two-wire sound-powered circuit is passive—the signal is generated by the user’s own voice and carried along the line without any external power source. The mouthpiece microphone converts spoken sound into a small electrical variation, this variation travels on the two-conductor line, and the earphone reproduces the sound from that current fluctuation. Because there’s no need for an external power supply, no battery or central power is required for operation. That’s why the statement describing them as not requiring electrical power to operate is best. The other options imply power sources that aren’t part of how sound-powered two-wire systems work, and they aren’t inherent traits of this setup.