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

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