试确定 $k$ 为何值时, 向量组 $\alpha_1, \alpha_2, \alpha_3$ 的秩为 2 ,
$$
\alpha_1=\left(\begin{array}{c}
1 \\
2 \\
-1 \\
1
\end{array}\right), \alpha_2=\left(\begin{array}{l}
2 \\
0 \\
k \\
0
\end{array}\right) \alpha_3=\left(\begin{array}{c}
0 \\
-4 \\
5 \\
-2
\end{array}\right) .
$$