A microwave transmitter sends waves at a metal plate. The waves hit the plate and are reflected back in the direction they came from. A microwave detector is then moved along the line PT. Points P, Q, R, S and T are the places where minima of intensity are recorded. The spacing between these points is found to be $15 \text{ mm}$. What is the frequency of the microwaves?
- A$5.0 \text{ GHz}$
- B$6.7 \text{ GHz}$
- C$10 \text{ GHz}$
- D$20 \text{ GHz}$