A transmitter sends a pulse of electromagnetic waves towards a reflector. The pulse is reflected back and reaches the transmitter again. A detector is placed at the transmitter. The outgoing pulse and the reflected pulse are shown on a cathode-ray oscilloscope (c.r.o.) as shown. The pulse takes $6.3\,\mu\text{s}$ to travel from the transmitter to the reflector. What is the time-base setting of the c.r.o.?
- A$2.1\,\mu\text{s}\,\text{cm}^{-1}$
- B$3.2\,\mu\text{s}\,\text{cm}^{-1}$
- C$4.2\,\mu\text{s}\,\text{cm}^{-1}$
- D$6.3\,\mu\text{s}\,\text{cm}^{-1}$