Two identical photocathodes receive light of frequencies $f_{1}$ and $f_{2}$ respectively. If the velocities of the photo-electrons emitted are $v_{1}$ and $v_{2}$ respectively,then:

• A
  $v_{1}^{2} - v_{2}^{2} = \frac{2h}{m} [f_{1} - f_{2}]$
• B
  $v_{1}^{2} + v_{2}^{2} = \frac{2h}{m} [f_{1} + f_{2}]$
• C
  $v_{1} + v_{2} = [\frac{2h}{m} (f_{1} + f_{2})]^{1/2}$
• D
  $v_{1} - v_{2} = [\frac{2h}{m} (f_{1} - f_{2})]^{1/2}$