In a pressure penstock 4500 m long, water is flowing at a velocity of 4 m/s. If the velocity of the pressure wave traveling in the pipe, due sudden complete closure of a valve at the downstream end, is given as 1500 m/s, what would be the period of oscillation in second under frictionless conditions? (a) 6 (b) 8 (c) 9 (d) 11