A cathode-ray oscilloscope (CRO) is being used to find the frequency of a sound wave. The diagram displays the waveform on the screen. The time-base setting is $5.0\,\text{ms div}^{-1}$. What is the most accurate estimate of the sound wave’s frequency?
- A$50\,\text{Hz}$
- B$71\,\text{Hz}$
- C$100\,\text{Hz}$
- D$143\,\text{Hz}$