Exploring Line Reflections in the Coordinate Plane - Barry

Interact with the following app for a few minutes. Be sure to drag point B, point C, the dashed line, and Lisa's picture around!
From what you've seen, what causes Lisa's image to coincide (lie right on top of) her original image (preimage)? Describe.
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– Melissa Barry, Tim Brzezinski

Information: Exploring Line Reflections in the Coordinate Plane - Barry