The refractive index of the material of a prism is $\sqrt{2}$ and the angle of the prism is $30^{\circ }$. One of the two refracting surfaces of the prism is made a mirror inwards, by silver coating. A beam of monochromatic light entering the prism from the other face will retrace its path (after reflection from the silvered surface) if its angle of incidence on the prism is:

1. A
   $30^{\circ }$
2. B
   $45^{\circ }$
3. C
   $60^{\circ }$
4. D
   Zero