Two plane mirrors are inclined to each other at some angle. A ray of light incident at $30^{\circ}$ on one mirror after reflection from the other mirror retracts its path. The angle between the mirror is :

Question

$(a)\;30^{\circ} \\ (b)\;45^{\circ} \\ (c)\;90^{\circ} \\ (d)\;20^{\circ}$