A square-wave trace is shown on the screen of a cathode-ray oscilloscope. The screen is overlaid with a grid made of $1\,\text{cm}$ squares. The time-base setting is $10\,\text{ms cm}^{-1}$. Estimate the frequency of the square wave.
- A$70\,\text{Hz}$
- B$140\,\text{Hz}$
- C$280\,\text{Hz}$
- D$1400\,\text{Hz}$