Complete the following sentence :
Ultrasound is of frequency ................

Explanation

**Step-by-Step Solution:**

1. **Understanding Ultrasound**: We need to identify what ultrasound is in terms of its frequency. Ultrasound refers to sound waves that are beyond the range of human hearing.

2. **Identifying the Audible Range**: The audible range for human beings is typically from 20 Hz to 20 kHz (20,000 Hz). This means that sounds within this frequency range can be heard by the average human ear.

3. **Determining the Frequency of Ultrasound**: Since ultrasound is defined as sound waves that are above the audible range, we need to find the frequency that exceeds 20 kHz.

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