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 distance $R$ from the centre of the Sun is <br> Where $\sigma$ is the Stefan's Contant :-

Question

( A ) $r^2 \sigma (t+273)^4/4 \pi R^2$
( B ) $r^2 \sigma (t+273)^4/R^2$
( C ) $16 \pi^2 r^2 \sigma t^4 / R^2$
( D ) $4 \pi r^2 \sigma t^4/R^2$