Precise Geometric Definitions from Undefined Terms Practice Test

A student claims that two lines are perpendicular because they intersect at a $$90°$$ angle. However, the teacher points out that this reasoning is incomplete based on the precise geometric definition. What additional condition must be verified to ensure the lines are truly perpendicular?

In a coordinate plane, points $$A(-3, 2)$$, $$B(1, 2)$$, and $$C(5, 2)$$ are given. A student argues that since all three points have the same $$y$$-coordinate, they form a line segment. Using precise geometric definitions, which statement best explains why this reasoning is insufficient?