A point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance at this portion varies with its distance r from the source as
(a) I∝ 1/r
(b) I ∝ 1/r2
(c) I ∝ 1/r3
(d) I ∝ 1/r4
