The equation of the parabola whose focus is the point (0, 0) and the tangent at the vertex is `x-y+1=0` is (a) `x^2+y^2-2x y-4x-4y-4=0` (b) `x^2+y^2-2x y+4x-4y-4=0` (c)`x^2+y^2+2x y-4x+4y-4=0` (d)`x^2+y^2+2x y-4x-4y+4=0`