Functions f, g, and h are shown on the graph.

Two nonlinear functions f vertex at (1, 6) and intercepts the y-axis at 5 units and h vertex at (6, 6) and intercepts the x-axis at 3.5 and 8.5 units. A linear function g passes diagonally with the interception of (0, 0).

Which of these functions have a domain of all real numbers?

A.
functions f and g only
B.
functions g and h only
C.
function f only
D.
functions f, g, and h