Assuming the sun to be a spherical body of radius $R$ at a temperature of $T \ K$,evaluate the total radiant power incident on the earth at a distance $r$ from the sun. (Where $r_0$ is the radius of the earth and $\sigma$ is Stefan's constant.)

• A
  $\frac{R^2 \sigma T^4}{r^2}$
• B
  $\frac{4 \pi r_0^2 R^2 \sigma T^4}{r^2}$
• C
  $\frac{\pi r_0^2 R^2 \sigma T^4}{r^2}$
• D
  $\frac{r_0^2 R^2 \sigma T^4}{4 \pi r^2}$