D1.2. A vector field S is expressed in rectangular coordinates as S = {125/ [(x − 1)²+(y−2)²+(z+1)²]}{(x − 1)ax + (y − 2)ay +(z+1)az}. (a) Evaluate S at P(2, 4, 3). (b) Determine a unit vector that gives the direction of S at P. (c) Specify the surface f(x, y, z) on which |S| = 1.