6. [6 pts] Which of the following are true 1. 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 Vf (x,y) = 2x. TH. [ I f(x, y)dyax = [, J. f(x,y)dxdy IV.V The gradient is always orthogonal to the level curves or surfaces. A. II and IV B. J. III and IV C. II and III D. Dand IV E. They are all truc