A can do a piece of work in 15 days working 6 hours per day and B can do it in 10 days working 5 hours per day. How long (in days) will they take to do it, working together for 9 hours per day?

A

$$6\frac{4}{7}$$

B

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

C

$$3\frac{4}{7}$$

D

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