A microphone, linked to a cathode-ray oscilloscope (CRO), detects a sound wave. The CRO screen shows a waveform, as illustrated. The time-base is adjusted to $20\,\mu\text{s div}^{-1}$. What is the frequency of the sound wave?
- A$15\,\text{Hz}$
- B$15\,000\,\text{Hz}$
- C$20\,000\,\text{Hz}$
- D$30\,000\,\text{Hz}$