Explanation: VHF marine radios operate on specific channels with designated transmit and receive frequencies. The USA-INT control selects between U.S. and International channel assignments. In **USA mode**, many channels (especially those used for communication with shore stations like the Coast Guard or commercial operations) are **duplex**. This means your radio transmits on one frequency and receives on a different frequency simultaneously. Other channels are **simplex**, transmitting and receiving on the same frequency. If you leave the radio on **INT (International) mode** in U.S. waters, some channels that are duplex under U.S. regulations may operate as **simplex** or use different duplex frequency pairs under international rules. The problem arises when your radio, in INT mode, transmits on a frequency that corresponds to the **receive** frequency of a U.S. station operating in USA mode on a duplex channel. The U.S. station will hear your transmission. However, because your radio is either listening on its transmit frequency (simplex in INT mode) or an international duplex receive frequency, it will not be listening on the frequency that the U.S. station is *transmitting* on for its reply. Therefore, you will hear nothing back. * **A) Your signals will be heard but other station replies will not be heard on certain channels.** This correctly describes the scenario where your transmit frequency aligns with their receive frequency, but your receive frequency does not align with their transmit frequency. * **B) You will be able to communicate with public correspondence stations on Channel 5.** Channel 5 is a duplex channel in USA mode; operating in INT mode would likely prevent proper communication due to the frequency mismatch. * **C) Other stations will not be able to hear your transmissions and you will not hear any signals.** This is incorrect; the issue is specifically about being heard but not hearing replies on certain channels, not a complete communication failure. * **D) You will not be able to operate on channel 13.** Channel 13 is a simplex channel for bridge-to-bridge. While channel assignments can vary, the primary and most problematic effect of the USA-INT switch is on duplex channels causing the "heard but not hear" issue.