Pipe A fills an empty tank in 4 hours and pipe B in 6 hours. If the two pipes are opened for one hour each alternatively with pipe A being opened first, then the tank will be filled (in hours) is

A

$$2\frac{3}{4}$$

B

$$3\frac{1}{2}$$

C

$$4\frac{2}{3}$$

D

$$5\frac{1}{4}$$