If $$\sigma$$ is the standard deviation of the observations $$x_1, x_2, ........., x_n$$ then the standard deviation of $$5x_1 + 7, 5x_2 + 7, ......, 5x_n + 7$$ is

A

$$5 \sigma + 7$$

B

$$\sqrt{25 \sigma^2 + 7}$$

C

$$5 \sigma$$

D

$$5 \sigma + \sqrt{7}$$