设 $A=\left(\alpha_1, \alpha_2, \alpha_3, \alpha_4\right)$ 为 4 阶正交矩阵，若矩阵
$$
B=\left(\begin{array}{l}
\alpha_1^T \\
\alpha_2^T \\
\alpha_3^T
\end{array}\right), \beta=\left(\begin{array}{l}
1 \\
1 \\
1
\end{array}\right), k \text { 为任意常数， }
$$
则线性方程组 $B x=\beta$ 的通解为
$\text{A.}$ $\alpha_2+\alpha_3+\alpha_4+k \alpha_1$
$\text{B.}$ $\alpha_1+\alpha_3+\alpha_4+k \alpha_2$
$\text{C.}$ $\alpha_1+\alpha_2+\alpha_4+k \alpha_3$
$\text{D.}$ $\alpha_1+\alpha_2+\alpha_3+k \alpha_4$