The diagram illustrates the screen of a cathode-ray oscilloscope (c.r.o.) showing a wave. The time-base on the c.r.o. is adjusted to $10\ \text{ms\ per division}$. What is the frequency of the wave?
- A$0.24\ \text{Hz}$
- B$4.2\ \text{Hz}$
- C$12\ \text{Hz}$
- D$24\ \text{Hz}$