Consider the function given below and pick one or more CORRECT statement(s) from the following choices.

$$ f(x)=x^3-\frac{15}{2} x^2+18 x+20 $$

Pick the CORRECT eigen value(s) of the matrix $[\mathrm{A}]$ from the following choices.

$$ [A]=\left[\begin{array}{ll} 6 & 8 \\ 4 & 2 \end{array}\right] $$

Consider a discrete random variable $X$ whose probabilities are given below. The standard deviation of the random variable is _________ (round off to one decimal place).

$$ \begin{array}{|c|c|c|c|c|} \hline x_1 & 1 & 2 & 3 & 4 \\ \hline P\left(X=x_i\right) & 0.3 & 0.1 & 0.3 & 0.3 \\ \hline \end{array} $$

The bacteria mainly responsible for crown corrosion in a sewer is $\qquad$