Light,leaving a Point Source,spreads Out So That the Apparent Brightness,b,of $\alpha$

Multiple Choice

Light,leaving a point source,spreads out so that the apparent brightness,b,of light per unit area varies with distance d according to which law ( $\alpha$ means "proportional to") ?

A) b $\alpha$ 1/d².
B) b = constant.
C) b $\alpha$ 1/d.
D) b $\alpha$ d².