A parabola has a vertex at open paren 0 comma 0 close paren The focus of the parabola is located at open paren 4 comma 0 close paren What is the equation of the directrix?

The focus at (4, 0) is 4 units to the right of the vertex (0, 0), so the parabola opens to the right. The directrix is perpendicular to the axis of symmetry and the same distance on the opposite side of the vertex, giving x = −4. Therefore, the directrix is choice D.

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