Extra exam question E4E10

WhyArcing contacts in a thermostatically controlled device, a defective doorbell transformer, and a malfunctioning illuminated sign can all create intermittent loud roaring or buzzing AC line interference. Intermittent loud roaring or buzzing AC line interference can come from several different sources: arcing contacts in a thermostatically controlled device, a defective doorbell or doorbell transformer nearby, or a malfunctioning illuminated advertising sign, each of which can generate this kind of noisy, intermittent electrical arcing, answer D. Since all three are genuine real-world sources of this specific interference pattern, no single one alone is the complete answer.