Well, a 2D object has only length and width, but a 3D object has length, width, and depth.
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Paper is a 3D object.
The cross section, of a 3D object, is the 2D shape made when a 2D plane cuts across the 3D object. Often "cross section" refers to the shape made when the plane is at right angles to an axis of the 3D object but this need not be the case.
A 2D shape is flat and only has a surface area whereas a 3D shape is an object that has surface area and volume.
a pyramid is a 3D objecta triangle is a 2D object
2D, a cube would be 3D