Explanation: The required bridge-to-bridge watch is primarily conducted on VHF-FM Channel 13 (156.650 MHz) in most areas of the continental United States. This is mandated by U.S. Coast Guard regulations (33 CFR Part 26) to facilitate direct communication between vessels, aiding in navigation, collision avoidance, and coordinating passing arrangements, especially in busy waterways. DSC on Channel 70 is used for Digital Selective Calling, enabling distress alerting, safety, and routine calling using digital signals, not continuous voice watch. VHF-FM Channel 16 is the international distress, safety, and calling channel, monitored by all vessels for initial contact or emergencies, but it is not the designated channel for routine bridge-to-bridge navigational communication. A vessel’s working frequency is utilized for general, non-urgent communications after initial contact on a calling channel, not for a mandatory safety watch.