If $V_x , V_m\: and\: V_r$ respectively are the speed of X-rays, microwaves and radio waves in air, then

Question

$\begin {array} {1 1} (a)\;V_x > V_m > V_r & \quad (b)\;V_x < V_m < V_r \\ (c)\;V_x = V_m > V_r & \quad (d)\;V_x = V_m = V_r \end {array}$