# Electromagnetic waves travel in a medium at a speed of $2 \times 10^8 m/s$. The relative permeability of the medium is $1$. Calculate the relative permittivity.
$\begin {array} {1 1} (a)\;2.25 & \quad (b)\;22.5 \\ (c)\;225 & \quad (d)\;0.225 \end {array}$