$A$ 为 4 阶实对称矩阵, 且 $A ^2+ A = O$, 若 $A$ 的秩为 3 , 则 $A$ 相似于
A. $\left(\begin{array}{llll}1 & & & \\ & 1 & & \\ & & 1 & \\ & & & 0\end{array}\right)$.
B. $\left(\begin{array}{llll}1 & & & \\ & 1 & & \\ & & -1 & \\ & & & 0\end{array}\right)$.
C. $\left(\begin{array}{llll}1 & & & \\ & -1 & & \\ & & -1 & \\ & & & 0\end{array}\right)$.
D. $\left(\begin{array}{llll}-1 & & & \\ & -1 & & \\ & & -1 & \\ & & & 0\end{array}\right)$.