The Standard Deviation for a Random Variable with Probability Density $f$

Multiple Choice

The standard deviation for a random variable with probability density function $f$ and mean $\mu$ is defined
$$\sigma = \left[ \int _ { - \infty } ^ { \infty } ( x - \mu ) ^ { 2 } f ( x ) d x \right] ^ { 1 / 2 }$$
Find the standard deviation for an exponential density function with mean $10 .$

A) 3.3
B) 2
C) 5
D) 2.5
E) 10