*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)  

During the day, the D region of the ionosphere becomes more ionized due to solar radiation. This increased ionization causes the D region to absorb radio signals, particularly those in the lower frequency HF bands like 40, 60, 80, and 160 meters. This absorption makes long-distance communication on these bands more difficult during daylight hours.

### Explanation:

1. **Correct Answer: C - The D region absorbs signals at these frequencies during daylight hours**
   - The D region is the lowest part of the ionosphere and is most active during the day due to solar radiation. It absorbs lower frequency HF signals, which reduces their strength and makes long-distance communication challenging.

2. **Why the Other Choices are Incorrect:**
   - **A: The F region absorbs signals at these frequencies during daylight hours**
     - The F region primarily refracts signals rather than absorbing them. It is responsible for long-distance propagation, especially at night when the D region's absorption decreases.
   - **B: The F region is unstable during daylight hours**
     - The F region is generally stable and effective for long-distance communication during both day and night, although its characteristics change with solar activity.
   - **D: The E region is unstable during daylight hours**
     - The E region can reflect signals, but it does not significantly absorb them. It is not the primary factor affecting long-distance communication on these bands during the day.

### Useful Rule of Thumb:
- **Daytime Absorption:** The D region causes significant absorption of lower HF frequencies during the day, making bands like 40, 60, 80, and 160 meters less effective for long-distance communication.