By the unique line postulate, you can draw only one segment, . Using the definition of , reflect BC over l. By the definition of reflection, C is the image of itself and is the image of B. Since reflections preserve , AC = BC.

A reflection preserves length, so the distance between the image points is the same as the distance between the original points. Here, C stays fixed on line l and B reflects to A, so segment BC reflects to segment AC. Therefore, AC = BC, and the missing word is “length.”

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