Which of the following functions represents exponential growth?
f(x)=5(0.9)xf(x) = 5(0.9)^xf(x)=5(0.9)x
f(x)=100(1.2)xf(x) = 100(1.2)^xf(x)=100(1.2)x
f(x)=2(1)xf(x) = 2(1)^xf(x)=2(1)x
f(x)=4−xf(x) = 4 - xf(x)=4−x