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:

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