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. `40kcal//(m^(2)-min)`
B. `80kcal//(m^(2)-min)`
C. `160kCal//(m^(2)-min)`
D. `320Kcal(m^(2)-min)`