A 3D scan is taken of a dodecahedron.

To do this a plane starts at one face and is slowly lowered until it gets to the opposite face always staying parallel to, and between, those two faces.

The picture above shows it when this process is half way completed.

As it is lowered, the cross section takes on a variety of polygons with different numbers of edges.

Which of the following shows this progression, i.e. the number of edges the cross section has, as it is lowered?

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The plane starts at the first face (5 sides) then proceeds down until it hits the first plane of vertices, at which point each planar vertex splits into two having 10 sides. Then it hits the next plane of vertices, at which point every other edge disappears, becoming a pentagon again. So it goes from 5 sides to 10 sides to 5 sides.