If light travels a distance $x$ in time $t_1$ sec in air and $10x$ distance in time $t_2$ sec in a certain medium,then find the critical angle of the medium.

• A
  $\sin ^{-1}\left(\frac{20 t_1}{t_2}\right)$
• B
  $\sin ^{-1}\left(\frac{10 t_1}{t_2}\right)$
• C
  $\sin ^{-1}\left(\frac{t_1}{t_2}\right)$
• D
  $\sin ^{-1}\left(\frac{t_2}{10 t_1}\right)$