If $$\sigma$$ is the standard deviation of $$x_1, x_2, ...... x_n$$, then the standard deviation of $$9 + 3x_1, 9 + 3x_2, ....., 9 + 3x_n$$ is

A

$$3 \sigma - 3$$

B

$$\sqrt{9 \sigma^2 + 3}$$

C

$$3 \sigma$$

D

$$3 \sigma + 9$$