Because it has a volume so therefore it is a 3d object whereas 2d shapes are normally flat and have no volume.
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A cylinder is not a 2D object because it has three dimensions - length, width, and height. It has a circular base and a curved surface, which give it depth. A 2D object would only have two dimensions and would lack any depth or thickness.
the cylinder has 2 2d shapes: the 2 circles on the end
Well, a 2D object has only length and width, but a 3D object has length, width, and depth.
a circle is a 2D shape while a cylinder is 3D shape
a pyramid is a 3D objecta triangle is a 2D 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.