A classification manual states: “All triangles are polygons.” A staff member checks an identification result, and the system displays: “This sign is a polygon.” He immediately says: “So this sign is a triangle.” What is wrong with this judgment?

Answer: C

• AThe reasoning is valid: since all triangles are polygons, and it is a polygon, it is naturally a triangle.✗ This mistakes “every member of a class has a feature” for “everything with the feature belongs to the class”, which is exactly the fallacy of the undistributed middle.
• BThe problem is that “polygon” means something different in the two statements; the wording shifts between them.✗ Wrong: both occurrences refer to the same geometric classification, so there is no shift in meaning or equivocation.
• CPolygons are not only triangles; the identified shape alone cannot establish that it belongs to the triangle category.✓ Correct: other shapes also share this common feature, so the feature alone cannot pin down the specific category — the middle term is undistributed.
• DThe problem is that the system identified only one sign; the sample size is too small to make a judgment.✗ Wrong: the number of identifications has nothing to do with whether one may infer from a broader category to a narrower one.