Quadratic functions q and w are graphed on the same coordinate grid. The vertex of the graph of q is 18 units below the vertex of the graph of w. Which pair of functions could have been used to create the graphs of q and w?
A. q(x) = 18x^2 and w(x) = x^2
B. q(x) = x^2 + 18 and w(x) = x^2
C. q(x) = -18x^2 and w(x) = x^2
D. q(x) = x^2 - 18 and w(x) = x^2