Looking at the surface of a smooth 3-dimensional object from the outside, which one of the following options is TRUE?

A smooth 3D object (a differentiable surface without sharp points or edges) can have regions with different local curvatures depending on its shape:

• It does not have to be concave everywhere (e.g., a standard sphere is strictly convex everywhere, so A is false).

• It does not have to be convex everywhere (e.g., a torus or a dumbbell shape has regions of saddle-like or concave curvature as viewed from the outside, so B is false).

• A smooth object can be convex in some regions and concave/saddle-shaped in others, making (C) true.

• A "smooth" surface by definition has no sharp edges or corners, making D false.

Thus, option (C) is the correct statement.