If a function f(x,y) has continuous second partial derivatives throughout an open region R, must the first-order derivatives of f be continuous on R? Give reasons for your answer. Which of the following conclusions follows from the given fact that f(x,y) has continuous second partial derivatives fxx. fxy, and fry throughout an open region R? Choose the correct answer below. OA. At every point of R. fxx fxy, and fyy are differentiable. OB. The functions f, and fy are differentiable at every point of R. OC. For some point (xo.Yo) in R, fxx = fxy = fyy- OD. The integral of fxx *fxy over the region R is equal to fx, and the integral of fxy fyy over the region R is equal to f..