The phase velocity of a plane wave given by $$E_x = E_o \cos(\omega t - \beta z)$$ with a frequency of 5.0 GHz and a wavelength in the material medium of 3.0 cm is:

A

$$3.0 \times 10^8$$ m/sec

B

$$1.5 \times 10^8$$ m/sec

C

$$5 \times 10^8$$ m/sec

D

None