The velocity of air in m/s is

Question

Figure shows the schematic for the measurement of velocity of air (density = 1.2 kg/m³) through a constant –area duct using a Pitot tube and a water-tube manometer. The differential head of water (density = 1000 kg/m³) in the two columns of the manometer is 10 mm. Take acceleration due to gravity as 9.8 m/s². The velocity of air in m/s is       

     (a) 6.4                                                                       (b) 9.0

      (c) 12.8                                                                    (d) 25.6