The correct response that identifies the first step in the construction of a perpendicular bisector is:
"A line segment has 2 end points. The left end point is A, and the right is B. A geometric compass is shown with the compass needle on point A and the pencil of the compass placed about two thirds of the way along line segment A B."
This step describes using a compass to create arcs from points A and B, which is the first action required to construct a perpendicular bisector.