Pipes A and C can fill an empty cistern in 7 and 10.5 hours, respectively while Pipe B can drain the filled cistern in 5.25 hours. If the three pipes are turned on together when the cistern is empty, how many hours will it take for the cistern to be $$\frac{2}{3}$$ full?

A

121

B

12

C

14

D

15.75