A sound wave with frequency $270\,\text{Hz}$ is displayed on a cathode-ray oscilloscope (CRO). The CRO waveform is shown. What is the time-base setting on the CRO?
- A$0.1\,\text{ms cm}^{-1}$
- B$1\,\text{ms cm}^{-1}$
- C$10\,\text{ms cm}^{-1}$
- D$100\,\text{ms cm}^{-1}$