Which of the following functions represents exponential decay?
f(x)=3(1.2)xf(x) = 3(1.2)^xf(x)=3(1.2)x
f(x)=2(0.9)xf(x) = 2(0.9)^xf(x)=2(0.9)x
f(x)=0.5(5)xf(x) = 0.5(5)^xf(x)=0.5(5)x
f(x)=10(1.01)xf(x) = 10(1.01)^xf(x)=10(1.01)x