Which of the following is the correct general form of Snell's law?

• A
  $n_{21} = \frac{\sin \theta_{2}}{\sin \theta_{1}}$
• B
  $\frac{n_{1}}{n_{2}} = \frac{v_{1}}{v_{2}}$
• C
  $n_{1} \sin \theta_{1} = n_{2} \sin \theta_{2}$
• D
  $n_{1} v_{1} = n_{2} v_{2}$