Two transparent media A and B are separated by a plane boundary. The speed of light in those media are $1.5\times 10^8\text{m/s}$ and $2.0\times 10^8\text{m/s}$, respectively. The critical angle for a ray of light for these two media is:

1. A
   ${\sin }^{-1}(0.500)$
2. B
   ${\sin }^{-1}(0.750)$
3. C
   ${\tan }^{-1}(0.500)$
4. D
   ${\tan }^{-1}(0.750)$