Light travels slower in oil than in air. If a light wave travels through the oil at a speed of 2 x 10^8 m/s and the wavelength of this light wave is 2 x 10^-3 m, what is the frequency of this wave?
A . 4 x 10^-24 hz
B . 1 x 10^5 hz
C . 1 x 10^11 hz
D . 4 x 10^5 hz
