In a Young’* double-slit experiment using slits of unequal widths, the intensities on the screen due to the slits are in the ratio 4 : 9 when the slits are used separately. When they are used together, the ratio of the intensity at a dark fringe to the intensity at a bright fringe on the screen will be
(a) 4 : 9
(b) 1 : 9
(c) 9 : 16
(d) 1 : 25
