The diagram illustrates a cathode-ray oscilloscope (c.r.o.) being used to determine the rotation rate of a flywheel. A small magnet is attached to the flywheel. Whenever the magnet moves past the coil, a voltage pulse is produced and then sent to the c.r.o. The width of the c.r.o. display is $10\ \text{cm}$. The flywheel is turning at $3000$ revolutions per minute. Which time-base setting will show pulses that are clearly separated on the screen?
- A$1\ \text{s cm}^{-1}$
- B$10\ \text{ms cm}^{-1}$
- C$100\ \mu\text{s cm}^{-1}$
- D$1\ \mu\text{s cm}^{-1}$