The interface AB between the two media A and B is shown in the figure. In the denser medium A, the incident ray PQ makes an angle of $30^{\circ}$ with the horizontal. The refracted ray is parallel to the interface. The refractive index of medium B w.r.t. medium A is : (A) $\frac{\sqrt{3}}{2}$ (B) $\frac{\sqrt{5}}{2}$ (C) $\frac{4}{\sqrt{3}}$ (D) $\frac{2}{\sqrt{3}}$