A lens if placed between a source of light and a wall. It forms images of area` A_(1)` and `A_(2)` on the wall for its two different positions. The area of the source or light is
A. `(A_1+A_2)/(2)`
B. `[(1)/(A_1)+(1)/(A_2)]^-1`
C. `sqrt(A_1A_2)`
D. `[(sqrtA_1+sqrt_2)/(2)]^2`