Monochromatic light of wavelength $\lambda$ is incident on an isolated metal sphere of radius $a$. The threshold wavelength is $\lambda_0$,which is greater than $\lambda$. How many photoelectrons will be emitted before the emission of photoelectrons stops?

• A
  $\frac{4\pi \epsilon_0 ahc}{e} \left[ \frac{1}{\lambda} + \frac{1}{\lambda_0} \right]$
• B
  $\frac{2\pi \epsilon_0 ahc}{e^2} \left[ \frac{1}{\lambda} - \frac{1}{\lambda_0} \right]$
• C
  $\frac{4\pi \epsilon_0 ahc}{e^2} \left[ \frac{1}{\lambda} - \frac{1}{\lambda_0} \right]$
• D
  $\frac{\pi \epsilon_0 ahc}{e^2} \left[ \frac{1}{\lambda} + \frac{1}{\lambda_0} \right]$