A can do a piece of work in 7 days by working 9 hours on each day and b can do it in 6 days by working 7 hours on each day. How long will they take to do it, working together for \[8\frac{2}{5}\] hours a day?

• A.  4 days                         
• B.  3 days
• C.  2 days                         
• D.  5 days