Two media A and B are separated by a plane boundary. The speed of light in medium A and B is $2 \times 10^8 \text{ ms}^{-1}$ and $2.5 \times 10^8 \text{ ms}^{-1}$ respectively. The critical angle for a ray of light going from medium A to medium B is : (A) $\sin^{-1} \frac{1}{2}$ (B) $\sin^{-1} \frac{4}{5}$ (C) $\sin^{-1} \frac{3}{5}$ (D) $\sin^{-1} \frac{2}{5}$