A microphone picks up a sound wave, and it is shown on the screen of a cathode-ray oscilloscope (CRO). The wave has a frequency of $2.5\,\text{kHz}$. What is the time-base setting of the CRO?
- A$0.1\,\text{ms cm}^{-1}$
- B$0.4\,\text{ms cm}^{-1}$
- C$0.1\,\text{s cm}^{-1}$
- D$0.4\,\text{s cm}^{-1}$