複選題4. Let f(x, y) = 4 + x³ + y³ - 3xy. Which of the following must be true? (A) f has a local maximum at (0,0). (B) f has a local minimum at (1,1). (C) At the point P(1,0), f have the maximum rate of change in the direction (1,-1). (D) The maximum rate of change at the point P(1,0) is.