The efficiency of A is thrice as that of B and efficiency of B is twice as that of C. If B alone can finish a work in 15 days, in how many days A and C together will complete that work?

A

$$4\frac{2}{7}$$

B

$$6\frac{1}{2}$$

C

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

D

$$4\frac{1}{3}$$