The radiant energy from the Sun incident normally at the surface of earth is `20kcal//m^(2)` min What would have been the radiant energy incident normally on the earth if the sun had a temperature twice of the present one ? .
A. `160 kcal//m^(2) min`
B. `40 kcal //m^(2)min`
C. `320 kcal //m^(2) min`
D. `80 kcal//m^(2)min`
