Dhiru can dig $$\frac{1}{a}$$ of a field in 20 hours. What fraction of the same field can Kaku dig in 20 hours if the two of them can dig the field in 60 hours, working together at their respective rates?

A

$$\frac{(a-3)}{a}$$

B

$$\frac{1}{3a}$$

C

$$\frac{3a}{(a-3)}$$

D

$$\frac{(a-3)}{3a}$$