The total radiant energy per unit area,normal to the direction of incidence,received at a distance $R$ from the centre of a star of radius $r$,whose outer surface radiates as a black body at a temperature $T \ K$ is given by

• A
  $\sigma \frac{r^2}{R^2} T^4$
• B
  $\frac{\sigma r^2}{4\pi R^2} T^4$
• C
  $\sigma \frac{r^4}{R^4} T^4$
• D
  $\sigma \frac{4\pi r^2}{R^2} T^4$