Which graph best represents a function with a range of all real numbers greater than or equal to -6?
Four graphs labeled A, B, C, and D showing quadratic functions on coordinate planes. Graph A shows a parabola opening upwards with vertex at y = -6. Graph B shows a parabola opening downwards with vertex at y = -7. Graph C shows a parabola opening upwards with vertex at y = -5. Graph D shows a parabola opening downwards with vertex at y = -6.