A transmitter sends out a pulse of electromagnetic waves towards a reflector. The pulse is then bounced back and arrives again at the transmitter. A detector is placed at the transmitter. The outgoing pulse and the returning pulse are shown on a cathode-ray oscilloscope (CRO), as illustrated. The pulse needs $6.3\,\mu\text{s}$ to move from the transmitter to the reflector. What is the CRO time-base setting?
- A$2.1\,\mu\text{s cm}^{-1}$
- B$3.2\,\mu\text{s cm}^{-1}$
- C$4.2\,\mu\text{s cm}^{-1}$
- D$6.3\,\mu\text{s cm}^{-1}$