Explanation: The sensitivity control of an auto-alarm receiver is crucial for its reliable operation, aiming to detect genuine distress signals while minimizing false alarms. * **A) Prevailing static at a given time:** Static, a form of natural radio noise, contributes to the overall background noise level. If the receiver's sensitivity is set too high (i.e., too sensitive), it will frequently trigger alarms due to ambient static, leading to false alerts. * **B) Radio interference at a given time:** Man-made radio interference, like static, also adds to the background noise floor. Similar to static, high sensitivity in the presence of interference will cause nuisance alarms. Therefore, the sensitivity must be adjusted just above the combined level of prevailing static and radio interference. This prevents the auto-alarm from constantly triggering on noise, ensuring it can effectively discriminate a genuine distress signal from the ambient background. * **C) Signal strength:** While the receiver needs to detect a signal, the sensitivity setting itself is not primarily determined by the expected signal strength. Instead, it's determined by the need to operate above the ambient noise floor (static and interference) to prevent false alarms, while remaining capable of detecting a legitimate signal that rises above that noise.