*Question E3A06:
What should be done to continue a long-distance contact when the MUF for that path decreases due to darkness?*  

*Correct Answer is  B: Switch to a lower frequency HF band*  

### AI Response: (may contain errors)  

To maintain a long-distance contact when the Maximum Usable Frequency (MUF) decreases due to darkness, the correct action is to switch to a lower frequency HF band. 

### Explanation of the Correct Answer:
- **Correct Answer: B - Switch to a lower frequency HF band**
  - As the MUF decreases, higher frequencies become less effective for long-distance communication. Lower frequencies can propagate better over long distances, especially during nighttime when the ionosphere's characteristics change. Lower frequencies tend to reflect off the ionosphere more effectively, allowing for better long-range communication.

### Explanation of Incorrect Answers:
- **A - Switch to a higher frequency HF band**
  - This is incorrect because higher frequencies are less effective when the MUF decreases. They may not reflect well off the ionosphere, leading to poor communication.

- **C - Change to an antenna with a higher takeoff angle**
  - This is incorrect because antennas with a higher takeoff angle are better suited for short-range communication. For long-distance contacts, a lower takeoff angle is preferred to allow signals to travel further by reflecting off the ionosphere.

- **D - Change to an antenna with greater beam width**
  - This is also incorrect. While a wider beam width can help in some situations, it does not address the fundamental issue of frequency propagation. The key to maintaining long-distance contacts is to use lower frequencies rather than changing the antenna's beam width.

### Summary:
- When the MUF decreases, switch to a lower frequency HF band to improve long-distance communication.
- Higher frequencies and antennas with higher takeoff angles are not suitable for long-distance contacts under these conditions. 

This understanding of frequency propagation and antenna characteristics is crucial for effective communication in amateur radio.