In the following table, $x$ is a discrete random variable and $p(x)$ is the probability density. The standard deviation of $x$ is
$$\begin{array}{|c|c|c|c|} \hline x & 1 & 2 & 3 \\ \hline p(x) & 0.3 & 0.6 & 0.1 \\ \hline \end{array}$$
- $0.18$
- $0.36$
- $0.54$
- $0.60$