原式=x2+ax-2y+7-(bx2-2x+9y-1),
=x2+ax-2y+7-bx2+2x-9y+1,
=(1-b)x2+(2+a)x-11y+8,
∴1-b=0,2+a=0,
解得b=1,a=-2,a+b=-1.
故选A.