A.0,0 B.0,1 C.1,0 D.1,1
A.0,0 B.0,1 C.1,0 D.1,1
A.x*y==0&&x+y!=0 B.x*y==0&&(x+y==0) C.x==0Y.=0 D.x*y=0X.y=0