Two identical photo-cathodes receive light of frequencies $f_1$ and $f_2$. If the velocities of the photo-electrons (of mass $m$) coming out are $v_1$ and $v_2$ respectively,then:

• A
  $v_1^2 - v_2^2 = \frac{2h}{m}(f_1 - f_2)$
• B
  $v_1 + v_2 = \left[ \frac{2h}{m}(f_1 + f_2) \right]^{1/2}$
• C
  $v_1^2 + v_2^2 = \frac{2h}{m}(f_1 + f_2)$
• D
  $v_1 + v_2 = \frac{2h}{m} \left[ \frac{2h}{m}(f_1 - f_2) \right]^{-1/2}$