If $$(1 + x + x^{2})^{n} = \sum_{r=0}^{2n} a_{r}x^{r}$$, then $$a_{0} + a_{2} + a_{4} + ... = $$

A

$$3^{n} + 1$$

B

$$3^{n} - 1$$

C

$$\frac{3^{n} + 1}{2}$$

D

$$\frac{3^{n} - 1}{2}$$