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Answer D to the third grade math problem (quality inspection problem), why? enclose herewith
The place you can't touch must be in the corner. Imagine a ball rolling once, only the corners can't be touched.

Did you choose C? If you choose C, it is the place that the circle can't touch after rotating around the center line of the Pentagon. Because the diameter of the circle is smaller than the length of the Pentagon, the circle can roll on the Pentagon, so C means that the circle cannot roll on the edge of the circle, so it is D.