A microphone is attached to a cathode-ray oscilloscope (CRO). The microphone detects a sound wave that has a constant frequency. The CRO screen is displayed. The time-base is adjusted to $1.0\,\text{ms cm}^{-1}$. What is the frequency of the sound wave?
- A$250\,\text{Hz}$
- B$500\,\text{Hz}$
- C$670\,\text{Hz}$
- D$4000\,\text{Hz}$