A semicircle?
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A polygon, but only if all the line segments are coplanar.
A Triangle :)
a closed, flat figure is a polygon. But it is only a polygon when it is not round.
A line is the only 1-dimensional figure, but only while it's a straight line. As soon as it turns or bends, it's then 2-dimensional.
If it is not straight, then it is not a line.