**A. 4 days**

B. 6 days

C. 8 days

D. 10 days

**Explanation**

If B takes 12 days to finish the work, then A takes 6 days to finish the same work.

If A can complete a work in x days and B can complete the same work in y days, then, both of them together can complete the work in x y/ x+ y days.

Therefore, the number of days taken by A and B together to finish the same work will be:

= 6 × 12/18= 4 days.