*Question E2E04:
Which of the following is characteristic of the FST4 mode?*  

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

### AI Response: (may contain errors)  

The correct answer to the question about the FST4 mode is **D: All these choices are correct**. Let's break down why this is the case:

1. **Why the correct answer is correct:**
   - **Four-tone Gaussian frequency shift keying (Choice A):** FST4 uses four distinct tones to represent data, which is a characteristic of Gaussian frequency shift keying (GFSK).
   - **Variable transmit/receive periods (Choice B):** FST4 allows for flexibility in the timing of transmissions and receptions, adapting to different conditions and requirements.
   - **Seven different tone spacings (Choice C):** The FST4 mode can utilize seven different tone spacings, which enhances its ability to transmit data effectively under varying conditions.

   Since all three statements accurately describe characteristics of the FST4 mode, the correct answer is indeed D.

2. **Why the other choices are incorrect:**
   - There are no incorrect choices in this case, as all the individual statements (A, B, and C) are true. Therefore, each choice contributes to the overall understanding of the FST4 mode.

3. **Useful rules of thumb:**
   - FST4 is a digital mode that is efficient for weak signal communication, making it popular in amateur radio for its robustness.
   - Understanding the modulation techniques, such as GFSK, is crucial for grasping how FST4 operates.

In summary, all the choices provided are correct characteristics of the FST4 mode, which is why the answer is D.