Assuming the sun to have a spherical outer surface of radius r, radiating like a black body at temperature t° C, the power received by a unit surface, (normal to the incident rays) at a distance R from the centre of the sun is:

• A

  $\frac{4{\mathrm{\pi r}}^2{\mathrm{\sigma t}}^4}{{\mathrm{R}}^2}$

• B

  $\frac{{\mathrm{r}}^2\mathrm{\sigma }{(\mathrm{t}+273)}^4}{4{\mathrm{\pi R}}^2}$

• C

  $\frac{16{\mathrm{\pi }}^2{\mathrm{r}}^2{\mathrm{\sigma t}}^4}{{\mathrm{R}}^2}$

• D

  $\frac{{\mathrm{r}}^2\mathrm{\sigma }{(\mathrm{t}+273)}^4}{{\mathrm{R}}^2}$