设 $\boldsymbol{A}=\left(\begin{array}{lll}a & 1 & 1 \\ 1 & a & a \\ 1 & 1 & a\end{array}\right)$ 可经初等列变换化成 $\boldsymbol{B}=\left(\begin{array}{lll}a & 1 & 1 \\ 1 & a & 1 \\ 1 & 1 & a\end{array}\right)$ ，则 $a$ 的取值范围为
A. $\{a \mid a \in \mathbf{R}, a \neq-2\}$ ．
B. $\{a \mid a \in \mathbf{R}, a \neq-2, a \neq-1\}$ ．
C. $\{a \mid a \in \mathbf{R}, a \neq 1, a \neq-1\}$ ．
D. $\{a \mid a \in \mathbf{R}, a \neq-1\}$ ．