*Question G3C05:
Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?*  

*Correct Answer is  C: The D region absorbs signals at these frequencies during daylight hours*  

### AI Response: (may contain errors)  

To understand why long-distance communication on the 40-, 60-, 80-, and 160-meter bands is more difficult during the day, we need to look at the behavior of the ionosphere and its different layers.

### Correct Answer: C
**The D region absorbs signals at these frequencies during daylight hours.**

- The D region of the ionosphere is primarily responsible for absorbing lower frequency signals, particularly in the HF (High Frequency) bands, which include the 40-, 60-, 80-, and 160-meter bands.
- During the day, solar radiation ionizes the D region more intensely, leading to increased absorption of radio waves. This makes long-distance communication on these bands challenging.

### Why the Other Choices are Incorrect:
- **A: The F region absorbs signals at these frequencies during daylight hours.**
  - The F region is actually beneficial for long-distance communication, especially during the day. It reflects higher frequency signals and is not primarily responsible for absorption at the frequencies in question.

- **B: The F region is unstable during daylight hours.**
  - While the F region can experience variations, it does not absorb signals. Instead, it can enhance propagation for higher frequencies. Thus, this statement does not explain the difficulties faced on the specified bands.

- **D: The E region is unstable during daylight hours.**
  - The E region can also reflect signals, but it is not the primary cause of absorption for the lower frequencies mentioned. Its instability does not directly impact the absorption characteristics of the D region.

### Summary
- The D region absorbs signals in the HF bands during the day, making long-distance communication difficult.
- The F and E regions do not absorb signals in the same way and can actually aid in propagation.

### Useful Rule of Thumb
- **Higher Frequencies = Better Daytime Propagation:** As frequency increases, the likelihood of successful long-distance communication during the day improves due to the behavior of the ionosphere.