Two capillary tubes A and B of equal radii r_a = r_b = r and equal lengths l_a = l_b = 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

  $\frac{\mathrm{Q}}{2}$

• B

  $\mathrm{Q}$

• C

  $2\mathrm{Q}$

• D

  $\frac{\mathrm{Q}}{\sqrt{2}}$