A horizontal tube has different cross sections at point A and B. The areas of cross section are a1 and a2, respectively, and pressures at these points are p1=ρgh1 and p2=ρgh2, where ρ is the density of liquid flowing in the tube and h1 and h2 are heights of liquid column in vertical tubes connected at A and B. If h1−h2=h, then the flow rate of the liquid in the horizontal tube is :