Two pipes A, B can fill a tank independently in 4, 6 hours respectively. Both the pipes are opened at the same instant and after $$x$$ hours pipe A is stopped. If the total time taken to fill the tank is $$3\frac{1}{2}$$ hours, then $$x$$ is

A

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

B

$$\frac{5}{3}$$

C

$$\frac{4}{3}$$

D

$$\frac{4}{5}$$