18.设矩阵A={a11a21,a12a22},B={a21+a11 a11,a22+a12 a12},p1={0 1,1 0},p2={1 1,0 1},则必有（ ）
A P1P2A=B
B P2P1A=B
C AP1P2=B
D AP2P1=B