A microphone picks up a sound wave. Its output is linked to the Y-input of a cathode-ray oscilloscope (c.r.o.). The trace on the c.r.o. is shown. The time-base is set at $0.20\,\text{ms}$ per division. What is the frequency of the sound wave?
- A$1000\,\text{Hz}$
- B$1250\,\text{Hz}$
- C$2000\,\text{Hz}$
- D$2500\,\text{Hz}$