or each of the following, find two unit vectors normal to the surface at an arbitrary point on the surface. a) The plane ax + by + cz = d, where a, b, c and d are arbitrary constants and not all of a, b, c are 0. (b) The half of the ellipse x2 + 4y2 + 9z2 = 36 where z > 0. (c)z=15cos(+y2). (d) The surface parameterized by r(u, v) = (Vu2 + 1 cos (), 2Vu2 + 1 sin (), u) where is any real number and 0< < 2T.