If light travels a distance $x$ in $t_1 \, s$ in air and $10x$ distance in $t_2 \, s$ in a medium,the critical angle of the medium will be

• A
  $\tan^{-1}\left(\frac{t_1}{t_2}\right)$
• B
  $\sin^{-1}\left(\frac{t_1}{t_2}\right)$
• C
  $\sin^{-1}\left(\frac{10t_1}{t_2}\right)$
• D
  $\tan^{-1}\left(\frac{10t_1}{t_2}\right)$