A differentiable function f(x,y) has a ________ point at a critical point (a,b) if in every open disk centered at (a,b) there are domain points (x,y) where f(x,y) > f(a,b) and domain points (x,y) where f(x,y).
(a) boundary
(b) fixed
(c) local maximum
(d) local minimum
(e) saddle