A cathode-ray oscilloscope (CRO) is being used to show a sound wave with frequency $2000\ \text{Hz}$. The CRO trace is shown. What is the time-base setting of the CRO?
- A$125\ \mu\text{s cm}^{-1}$
- B$250\ \mu\text{s cm}^{-1}$
- C$500\ \mu\text{s cm}^{-1}$
- D$1000\ \mu\text{s cm}^{-1}$