Audio signal cannot be transmitted because

Options

A. the signal has more noise.
B. the signal cannot be amplified for distance communication.
C. the transmitting antenna length is very small to design.
D. the transmitting antenna length is very large and impracticable.

Explanation

To solve the question "Audio signal cannot be transmitted because," we need to analyze the characteristics of audio signals and the requirements for transmission.

### Step-by-Step Solution:

1. **Understanding Audio Signals**:
- Audio signals are sound waves that typically have frequencies ranging from 20 Hz to 20,000 Hz (20 kHz). This range is what the human ear can hear.

2. **Evaluating the Options**:
- We need to consider the given options to determine why audio signals cannot be transmitted effectively.

3. **Option Analysis**:
- **Option 1**: "It cannot be transmitted because the signal has more noise."
- This is incorrect. While noise can affect transmission, it is not the primary reason audio signals cannot be transmitted directly.

- **Option 2**: "Signal cannot be amplified for distance communication."
- This is also incorrect. Audio signals can be amplified; the issue lies elsewhere.

- **Option 3**: "Transmitting antenna length is very small to design."
- This is incorrect as well. The problem is not that the antenna is too small.

- **Option 4**: "Transmitting antenna length is very large and impracticable."
- This is the correct option. For effective transmission of low-frequency signals (like audio), the antenna size must be proportional to the wavelength, which can be very large and impractical for audio frequencies.

4. **Conclusion**:
- Therefore, the correct answer is that audio signals cannot be transmitted because the transmitting antenna length is very large and impracticable.

### Final Answer:
Audio signal cannot be transmitted because the transmitting antenna length is very large and impracticable.