A cathode-ray oscilloscope (CRO) is employed to show the trace produced by a sound wave. The time-base is adjusted to $5\,\mu\text{s mm}^{-1}$. What is the frequency of the sound wave?
- A$6.7\,\text{Hz}$
- B$67\,\text{Hz}$
- C$6.7\,\text{kHz}$
- D$67\,\text{kHz}$