A horizontal ray of light passes through a prism of \[\mu =1.5\] whose apex angle is \[4{}^\circ \] and then strikes a vertical mirror M as shown. For the ray after reflection to become horizontal, the mirror must be rotated through an angle of:

• A. \[{{2}^{o}}\]
• B. \[{{3}^{\text{o}}}\]
• C. \[{{4}^{\text{o}}}\]
• D. \[{{1}^{\text{o}}}\]