A plane wave propagating in the dielectric medium has an electric field given as $$E_{x} = E_{o} \cos (2.6 \times 10^{10}t -100z)$$. The phase velocity of plane wave is :

A

$$10^{6} m/sec$$

B

$$2.6 \times 10^{8} m/sec$$

C

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

D

$$4.0 \times 10^{8} m/sec$$