A ray of light is incident upon a parallel sided transparent slab of thickness 9 cm at an angle of incidence ${60}^0$ .  If the angle of refraction is ${30}^0$ , the lateral displacement of the light ray is

• A

  $\sqrt{3 } cm$

• B

  $3\sqrt{3} cm$

• C

  $3 cm$

• D

  $\frac{2}{\sqrt{3}} cm$