设 $A =\left[\begin{array}{lll}1 & 2 & 0 \\ 2 & 1 & 0 \\ 0 & 0 & 1\end{array}\right]$ ，则下列矩阵中与 $A$ 合同的是 $(\quad)$ ．
$\text{A.}$ $\left[\begin{array}{lll}1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1\end{array}\right]$
$\text{B.}$ $\left[\begin{array}{ccc}1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & -1\end{array}\right]$
$\text{C.}$ $\left[\begin{array}{ccc}1 & 0 & 0 \\ 0 & -1 & 0 \\ 0 & 0 & -1\end{array}\right]$
$\text{D.}$ $\left[\begin{array}{ccc}-1 & 0 & 0 \\ 0 & -1 & 0 \\ 0 & 0 & -1\end{array}\right]$
$\text{E.}$
$\text{F.}$