Assuming the sun to have a spherical outer surface of radius $r$ radiating like a black body at temperature $t^{\circ} \mathrm{C}$, the power received by a unit surface (normal to the incident rays) at a distance $R$ from the centre of the sun is ( $\sigma$ is stefan's constant)
(a) $4 \pi r^{2} \sigma t^{4}$
(b) $\frac{r^{2} \sigma(t+273)^{4}}{4 \pi R^{2}}$
(c) $\frac{16 \pi^{2} r^{2} \sigma t^{4}}{R^{2}}$
(d) $\frac{r^{2} \sigma(1+273)^{4}}{R^{2}}$