Consider the following statements about the effect of different values of a on the function y = ab^x. Which one of the statements, if any, is incorrect?

A. Values of a such that |a| > 1 will vertically stretch the graph of y = b^x.
B. Values of a such that 0 < |a| < 1 will vertically compress the graph of y = b^x.
C. Negative values of a will reflect y = b^x across the y-axis.
D. All of the above statements are correct.