Which of the following statements is always true about the function y = a · b^x?

A. When a > 0 and b > 1, the function models exponential decay.
B. When a > 0 and b > 1, the function models exponential growth.
C. When a > 0 and 0 < b < 1, the function models exponential growth.
D. When a < 0 and 0 < b < 1, the function models exponential decay.