*Question E2C10:
Why do DX stations often transmit and receive on different frequencies?*  

*Correct Answer is  D: All these choices are correct*  

### AI Response: (may contain errors)  

DX stations often transmit and receive on different frequencies, a practice known as "split operation." Let's explore why this is beneficial:

1. **Correct Answer: D: All these choices are correct**

   - **A: Because the DX station may be transmitting on a frequency that is prohibited to some responding stations**  
     Some stations may not have permission to transmit on certain frequencies due to licensing restrictions. By using split operation, the DX station can transmit on a frequency that is accessible to all, while receiving on a different frequency that is open to more stations.

   - **B: To separate the calling stations from the DX station**  
     By using different frequencies for transmitting and receiving, the DX station can reduce congestion on the calling frequency. This separation helps manage the pile-up of stations trying to contact the DX station, making it easier to hear individual responses.

   - **C: To improve operating efficiency by reducing interference**  
     Split operation minimizes interference by spreading out the signals over different frequencies. This reduces the chance of overlapping transmissions, which can make communication clearer and more efficient.

2. **Why the other choices are incorrect:**

   - Each individual choice (A, B, or C) only addresses one aspect of why split operation is used. The correct answer, D, encompasses all these reasons, making it the most comprehensive choice.

3. **Useful Rules of Thumb:**

   - **Split Operation**: This is a common technique in DXing to manage large numbers of calling stations and ensure clear communication.
   - **Frequency Management**: Always check band plans and licensing restrictions to ensure compliance when operating split.

Understanding these concepts will help you appreciate the strategic use of split operation in amateur radio, especially during DX contests and rare station activations.