Two capillary tubes A and B of equal radii ra = rb = r and equal lengths la = lb = l are held horizontally. When the same pressure difference P is maintained across each tube, the rate of flow of water in each is Q. If the tubes are connected in series and the same pressure difference P is maintained across the combination, the rate of flow through the combination will be
A series combination of two identical capillary tubes, each of length l and radius r, is equivalent to a tube of length 2l and radius r.
Since Q is inversely proportional to l, if l is doubled, Q becomes half. Hence the correct choice is (a).