When reflecting a figure across the x-axis, why does the y-value always change signs?

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Step-by-step explanation:

When you reflect a point across the y-axis, the y-coordinate remains the same, but the x-coordinate is transformed into its opposite (its sign is changed). Notice that B is 5 horizontal units to the right of the y-axis, and B' is 5 horizontal units to the left of the y-axis.

not sure but it may be b, as i said i'm not sure.