Which statement correctly describes ultrasound?
- Alongitudinal waves with a frequency greater than $20\,000\ \text{Hz}$
- Blongitudinal waves with a frequency less than $20\ \text{Hz}$
- Ctransverse waves with a frequency greater than $20\,000\ \text{Hz}$
- Dtransverse waves with a frequency less than $20\ \text{Hz}$