A student employs a cathode-ray oscilloscope (c.r.o.) to determine the period of a signal. She adjusts the time-base on the c.r.o. to $5\ \text{ms cm}^{-1}$ and sees the trace shown. The trace measures $10.0\ \text{cm}$ in length. Determine the period of the signal.
- A$7.1 \times 10^{-6}\ \text{s}$
- B$1.4 \times 10^{-5}\ \text{s}$
- C$7.1 \times 10^{-3}\ \text{s}$
- D$1.4 \times 10^{-2}\ \text{s}$