Consider function f (x, y. z) = x y - 2vez + xz2 - 6y. At point (2, -1, 3), depending on the direction given by a vector v, the directional derivative f, (2, -1, 3) has different values. The largest value of fv (2, -1, 3) is ( a) 15 b) 21 O c) 7 O d) 9 O e) 18 Of) 11