6. [6 pts] Which of the following are true I.V If fis a differentiable function of two variables, and has a local minimum at (a, b), then Vf(a,b) = If f(x,y) = x2, then V/ (x,y) = 2x. [' I, f(x, y)dyax = [" ], f(x,y)dxdy IV.V The gradient is always orthogonal to the level curves or surfaces, A. Iland IV B. J. III and IV C. II and III D. and IV E. They are all truc