A tank is filled in 3 hours by three pipes A, B and C. Pipe A is 6 times as fast as pipe C and 2 times as fast as pipe B. How many hours will pipes B and C take to fill the tank?

A

$$7frac{4}{5}$$

B

7

C

$$7frac{1}{4}$$

D

$$7frac{1}{2}$$