Assertion : Humans cannot hear the sound of ultrasonic frequency.
Reason : Human ear can only hear sound of fre-quency range 20 Hz to 20000 Hz.

Answer

To analyze the assertion and reason provided in the question, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Humans cannot hear the sound of ultrasonic frequency."
- We need to determine if this statement is true or false.

2. **Defining Ultrasonic Frequency**:
- Ultrasonic frequencies are defined as sound waves that have a frequency greater than 20,000 Hz (20 kHz).
- Therefore, any sound above this frequency is considered ultrasonic.

3. **Understanding the Human Hearing Range**:
- The human ear can typically hear sounds in the frequency range of 20 Hz to 20,000 Hz.
- This means that sounds below 20 Hz are infrasonic, and sounds above 20,000 Hz are ultrasonic.

4. **Evaluating the Assertion**:
- Since ultrasonic frequencies are above the upper limit of human hearing (20,000 Hz), it is correct to say that humans cannot hear ultrasonic sounds.
- Thus, the assertion is true.

5. **Understanding the Reason**:
- The reason states that "Human ear can only hear sound of frequency range 20 Hz to 20,000 Hz."
- This statement accurately describes the audible range for humans.

6. **Evaluating the Reason**:
- Since the reason correctly describes the limits of human hearing, it is also true.

7. **Conclusion**:
- Both the assertion and the reason are true. The assertion is correct because humans cannot hear ultrasonic frequencies, and the reason is correct as it accurately describes the hearing range of humans.

### Final Answer:
- **Assertion**: True
- **Reason**: True