To do a certain work, the ratio of the efficiencies of A and B is 7:6. Working together, they can complete the same work in 21 days. What part of the same work will be completed by B alone in 26 days ?

A

$$\frac{5}{8}$$

B

$$\frac{4}{7}$$

C

$$\frac{5}{6}$$

D

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