Question 13 2 pts (1,2) is a critical point of the function f(x,y). Assuming the partial derivatives of f exist at (1,2), which of the following must be true? You may select one or more correct answers. Of (1,2) =0 Vf(1,2) = 0 Of xx(1,2) > 0 fy(1,2) =0 Of x(1,2) =0