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 parabolas on coordinate planes. Graph A shows an upward-opening parabola with vertex at (-3, 1). Graph B shows a downward-opening parabola with vertex at (-2, -7). Graph C shows an upward-opening parabola with vertex at (1, -6). Graph D shows a downward-opening parabola with vertex at (-3, -1).