设 $\boldsymbol{A}, \boldsymbol{B}$ 均为 3 阶矩阵, 则必有
A. $r(\boldsymbol{A}, \boldsymbol{A} \boldsymbol{B})=r(\boldsymbol{A})$.
B. $r(\boldsymbol{A}, \boldsymbol{B A})=r(\boldsymbol{A})$.
C. $r\left(\begin{array}{c}\boldsymbol{A} \\ \boldsymbol{A B}\end{array}\right)=r(\boldsymbol{A})$.
D. $r(\boldsymbol{A B})=r(\boldsymbol{B A})$.