A cathode-ray oscilloscope (c.r.o.) is being used to find the frequency of a sound wave. The diagram displays the waveform on the screen. The time-base setting is $5.0\,\text{ms/div}$. Determine the frequency of the sound wave.
- A$57\,\text{Hz}$
- B$71\,\text{Hz}$
- C$114\,\text{Hz}$
- D$143\,\text{Hz}$