When a photosensitive surface is irradiated by light of wavelength $\lambda_1$ and $\lambda_2$,the maximum kinetic energies of the emitted photoelectrons are $E_1$ and $E_2$ respectively. The work function of the photosensitive surface is:

• A
  $\frac{\lambda_1 E_1 - \lambda_2 E_2}{\lambda_2 - \lambda_1}$
• B
  $\frac{\lambda_1 E_1 + \lambda_2 E_2}{\lambda_2 - \lambda_1}$
• C
  $\frac{\lambda_1 E_2 - \lambda_2 E_1}{\lambda_2 - \lambda_1}$
• D
  $\frac{\lambda_1 E_2 + \lambda_2 E_1}{\lambda_2 - \lambda_1}$