A plane wave whose electric field is given by Eᵢ = 100 cos(omegat - 6pi*x)a_z passes normally from a material having epsilonᵣ = 4, muᵣ = 1, and sigma. What is the wave's velocity in the material?

a) 3 * 10⁸ / 2 m/s
b) 3 * 10⁸ / sqrt(4) m/s
c) 3 * 10⁸ / sqrt(5) m/s
d) 3 * 10⁸ / sqrt(6) m/s