A communications system employs microwaves that have a wavelength of $15\,$cm. The speed of light is $3.0 \times 10^8\,\text{m s}^{-1}$. What is the frequency of these microwaves?
- A$2.0 \times 10^7\,$Hz
- B$4.5 \times 10^7\,$Hz
- C$2.0 \times 10^9\,$Hz
- D$4.5 \times 10^9\,$Hz