若圆x2+y2-ax+2y+1=0与圆x2+y2=1关于直线y=x-1对称,过点C(-a,a)的圆P与y轴相切,则圆心P的轨迹方程为()
A.y2-4x+4y+8=0
B.y2-2x-2y+2=0
C.y2+4x-4y+8=0
D.y2-2x-y-1=0