Two plane mirrors $AB$ and $AC$ are inclined at an angle $\theta = 20^{\circ}$. $A$ ray of light starting from point $P$ is incident at point $Q$ on the mirror $AB$,then at $R$ on mirror $AC$,and again at $S$ on $AB$. Finally,the ray $ST$ goes parallel to mirror $AC$. The angle which the ray makes with the normal at point $Q$ on mirror $AB$ is: (in $^{\circ}$)

• A
  $20$
• B
  $30$
• C
  $40$
• D
  $60$