A group of ten persons can do a particular task in 10 days. Another group can do the same job in 20 days. In how many days will the job be finished if the groups works together (each group does part of the task in proportion to its total ability):

A

$$5\frac{1}{3}$$

B

$$6\frac{2}{3}$$

C

$$7\frac{1}{3}$$

D

$$8\frac{1}{2}$$