When a point light source, of power E emitting   monochromatic light of wavelength $\lambda$ is kept at a distance r from a photosensitive surface of work -function $\varphi$ and area s, then number of photons striking the surface per unit time is 

• A

  $\frac{\mathrm{W\lambda s}}{4{\mathrm{\pi hcr}}^2}$

• B

  $\frac{4\mathrm{\pi Ws}}{{\mathrm{hc\lambda r}}^2}$

• C

  $\frac{hc\lambda }{4\pi Ws{r}^2}$

• D

  $\frac{{\mathrm{W}}^2\mathrm{\lambda s}}{4{\mathrm{\pi hcr}}^2}$