Explanation: The auto-alarm system is designed to detect a specific international distress signal pattern, typically a series of 4-second dashes followed by 1-second spaces on designated frequencies. When the bell rings, it indicates the system has detected this pattern or something sufficiently similar to trigger an alert. A) A true alarm signal is the genuine distress signal transmitted by a station in an emergency. Detecting these is the primary purpose of the auto-alarm system. B) A false alarm signal can occur when the system is triggered by interference, equipment malfunction, or other transmissions that coincidentally mimic the distress pattern. Auto-alarm systems are designed to be highly sensitive and err on the side of caution, meaning they can activate even when no true distress is present. Therefore, the ringing of the bell means the system has processed incoming radio signals and identified them as a potential alarm, regardless of whether that alarm is genuine or a false positive.