Assuming the sun to have a spherical outer surface of radius $r$,radiating like a black body at temperature $t^{\circ} C$,the power received by a unit surface (normal to the incident rays) at a distance $R$ from the centre of the sun is,where $\sigma$ is the Stefan's constant.

• A
  $\frac{r^2 \sigma (t + 273)^4}{4\pi R^2}$
• B
  $\frac{16\pi^2 r^2 \sigma t^4}{R^2}$
• C
  $\frac{r^2 \sigma (t + 273)^4}{R^2}$
• D
  $\frac{4\pi r^2 \sigma t^4}{R^2}$