Triangle PQR is reflected across a line, P = (4,–5), and P’ = (–4, –5). What is the line of reflection?

A reflection line is the perpendicular bisector of the segment joining a point and its image. P(4, −5) and P′(−4, −5) have the same y-coordinate, so the segment between them is horizontal; its midpoint is (0, −5). The perpendicular bisector is the vertical line x = 0, which is the y-axis (choice B).

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