If f(x) is an even function and (6, 8) is one the points on the graph of f(x), which reason explains why (–6, 8) must also be a point on the graph?

a. since the function is even, the output of a negative x-value and its opposite is the same.
b. since the function is even, the output of a negative y-value and its opposite is the same.
c. the graph has rotational symmetry, so the point will be reflected across the y-axis.
d. the graph has rotational symmetry, so the point will be rotated 90 degrees about the origin.