If $$S_n = \sum_{r=0}^n \frac{1}{{}^n C_r}$$ and $$t_n = \sum_{r=0}^n \frac{r}{{}^n C_r}$$, then $$\frac{t_n}{S_n}$$ is equal to

A

$$\frac{1}{2}n$$

B

$$\frac{1}{2}n - 1$$

C

$$n - 1$$

D

$$\frac{2n - 1}{2}$$