When light propagates through a material medium of relative permittivity `` \epsilon_{ r } `` and relative permeability `` \mu_{ r ^{\prime}} `` the velocity of light, `` v `` is given by : ( `` c `` - velocity of light in vacuum)

(A) `` v=\frac{ c }{\sqrt{\epsilon_{ r } \mu_{ r }}} ``
(B) `` v= c ``
(C) `` v=\sqrt{\frac{\mu_{ r }}{\epsilon_{ r }}} ``
(D) `` v=\sqrt{\frac{\epsilon_{T}}{\mu_{r}}} ``