12. 设 $A, B$ 均为 $n$ 阶方阵,满足$\text{rank} \begin{bmatrix} A \\ B \end{bmatrix} = \text{rank}B,$ 且方程 $XA = B$ 有解。若 $\operatorname{rank} A = k$,则$\text{rank} \begin{bmatrix} A & O \\ B & E \end{bmatrix} =\underline{\hspace{2cm}}.$
12. 设 $A, B$ 均为 $n$ 阶方阵,满足$\text{rank} \begin{bmatrix} A \\ B \end{bmatrix} = \text{rank}B$ ,且方程 $XA = B$ 有解。若 $\operatorname{rank} A = k$,则$\text{rank} \begin{bmatrix} A & O \\ B & E \end{bmatrix} =\underline{\hspace{2cm}}.$