Two pipes A and B can fill an empty tank in 6 hours and 8 hours respectively. After opening both of them for x hours pipe B is closed and pipe A filled the rest of the tank in 4 hours, then x =

A

$$\frac{2}{3}$$

B

$$\frac{7}{8}$$

C

$$\frac{8}{7}$$

D

$$\frac{9}{7}$$