Pipes A, B and C can fill a tank in 10, 15 and 18 hours, respectively. A and C are opened for 5 hours, then only pipe A is closed and B is opened at the same time (with C). The total time taken to fill the tank completely is:

A

3\frac{2}{3} hours

B

6\frac{9}{11} hours

C

5\frac{8}{11} hours

D

2\frac{1}{3} hours