A column of mercury of length h=10 cm is contained in the middle of a narrow horizontal tube of length 1 m, closed at both ends. The air in both halves of the tube is under a pressure of P0=76 cm of mercury. The tube is now slowly made vertical. The distance moved by mercury will be approximately
(a) 4.5 cm
(b) 3.0 cm
(c) 2.5 cm
(d) 1.2 cm