Two Mirrors Are Joined Together Along a Common Side,the Planes $\degree$

Multiple Choice

Two mirrors are joined together along a common side,the planes of the mirrors separated by the angle è.If the joined mirrors can be used as a retroreflector when a beam of light strikes both surfaces (one after the other) ,what is the value of è?

A) 30 $\degree$
B) 45 $\degree$
C) 90 $\degree$
D) 135 $\degree$