Pipes X,Y and Z are fitted to a pank. X and Y can fill a tank in 18 hours and 24 hours respectively. Z is an emptying pipe which can empty the tank in 9 hours. All the pipes are opened simultaneously when the tank is half full. The tank would be
A. filled in 72 hours
B. empitied in 72 hours
C. filled in 36 hours.
D. emptied in 36 hours