A can do a piece of work in 12 days for 6 hours per day, and B can do it in 8 days for 7 hours per day. How long will they take to do the work, working together, for 9 hours a day?

A

4 days

B

$$\frac{5}{2}$$ days

C

$$\frac{9}{2}$$ days

D

$$\frac{7}{2}$$ days