Which of the following statements best describes the effect of replacing the graph of y = f(x) with the graph of y = f(x + 6)?
A. The graph of y = f(x) will shift up 6 units.
B. The graph of y = f(x) will shift down 6 units.
C. The graph of y = f(x) will shift left 6 units.
D. The graph of y = f(x) will shift right 6 units.