$$
\left(x_1, x_2, x_3\right)\left(\begin{array}{lll}
a_{11} & a_{12} & a_{13} \\
a_{12} & a_{22} & a_{23} \\
a_{13} & a_{23} & a_{33}
\end{array}\right)\left(\begin{array}{l}
x_1 \\
x_2 \\
x_3
\end{array}\right)
$$
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$