A为n阶方阵，若存在n阶方阵B，使AB=BA=A，则（ ）。\n \n \n \n \n \n \n

\n \n \n \n A.\n B为单位矩阵\n

\n \n

\n \n \n \n B.\n B为零方阵\n

\n \n

\n \n \n \n C.\n

\n \n \n

\n \n \n \n D.\n 不一定\n