设 $\boldsymbol{A}=\left(\begin{array}{ll}1 & 2 \\ 2 & 1\end{array}\right)$ ，则在实数域上与 $\boldsymbol{A}$ 合同矩阵为
A
$\left[\begin{array}{cc}-2 & 1 \\ 1 & -2\end{array}\right]$ ．
B
$\left(\begin{array}{cc}2 & -1 \\ -1 & 2\end{array}\right)$ ．
C
$\left(\begin{array}{ll}2 & 1 \\ 1 & 2\end{array}\right)$ ．
D
$\left(\begin{array}{cc}1 & -2 \\ -2 & 1\end{array}\right)$ ．
E
F