In air, the speed of sound is $330\,\text{m s}^{-1}$. Which sound is regarded as ultrasound?
- Aa sound with a wavelength of $250\,\text{cm}$
- Ba sound with a wavelength of $25\,\text{cm}$
- Ca sound with a wavelength of $2.5\,\text{cm}$
- Dnone of the above