二次型 $f\left(x_1, x_2, x_3\right)=x_1^2+2 x_2^2+3 x_3^2+4 x_1 x_2+6 x_2 x_3$ 的矩阵为
A. $\begin{array}{rrl}1 & 2 & 0 \\ (2 & 2 & 3) \\ 0 & 3 & 3\end{array}$
B. $\begin{array}{rrl}1 & 2 & 0 \\ (2 & 2 & 3) \\ 0 & 3 & 3\end{array}$
C. $\begin{array}{rrl}1 & 2 & 3 \\ (2 & 2 & 3) \\ 3 & 3 & 3\end{array}$
D. $\begin{array}{rrl}1 & 2 & 3 \\ (2 & 2 & 0) \\ 3 & 0 & 3\end{array}$