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