A and B can do a work in 21 days and 14 days, respectively. They started the work by working together, but after 5 days, due to some reasons, B left the work and A completed the remaining work. The whole work was completed in:

A

13 days

B

$$13\frac{1}{2}$$ days

C

$$10\frac{1}{2}$$ days

D

$$12\frac{1}{2}$$ days