When light of wavelength $\lambda$ is incident on a photosensitive surface,photons of power $P$ are emitted. The number of photons $n$ emitted in $t$ seconds is (where $h$ = Planck's constant,$c$ = velocity of light in vacuum).

• A
  $\frac{h c}{P \lambda t}$
• B
  $\frac{P \lambda t}{h c}$
• C
  $\frac{P \lambda}{h t c}$
• D
  $\frac{h P}{\lambda t c}$