The correct response is:
The legs of similar triangles used at different points along the line represent the rise and run of the line, or its slope. The corresponding sides of similar triangles are proportional, so the slope can be shown as the same using any two points along the line.
This explanation highlights how the proportionality of the sides of similar triangles demonstrates that the ratio of rise (change in y) to run (change in x) remains constant, illustrating that the slope is the same between any two distinct points on a nonvertical line.