Explanation: The Dual Watch (DW) function allows a marine VHF radio to monitor a selected working channel while periodically scanning or "peeking" at another important channel, most commonly Channel 16. The radio momentarily switches to Channel 16 to listen for distress calls or hails, then quickly returns to the chosen working channel. This ensures you can communicate on a preferred channel while still fulfilling the regulatory requirement to monitor the international distress and calling channel. Option A is incorrect because "at the same time" implies simultaneous listening, which is not how Dual Watch (on a single receiver radio) operates. Ch-70 is for Digital Selective Calling (DSC), which uses a dedicated receiver in modern radios, not typically part of the DW function for voice. Option B describes general scanning, not the specific two-channel monitoring of Dual Watch. Therefore, option D accurately describes the primary purpose and operation of the Dual Watch function.