The figure depicts the display on a cathode-ray oscilloscope (c.r.o.) showing a wave. The time-base of the c.r.o. is adjusted to $10\,\text{ms/div}$. What is the frequency of this wave?
- A$0.24\,\text{Hz}$
- B$4.2\,\text{Hz}$
- C$12\,\text{Hz}$
- D$24\,\text{Hz}$