Identify which function represents exponential decay.
f(x)=5(1.2)xf(x) = 5(1.2)^xf(x)=5(1.2)x
f(x)=2(0.9)xf(x) = 2(0.9)^xf(x)=2(0.9)x
f(x)=0.5(3)xf(x) = 0.5(3)^xf(x)=0.5(3)x
f(x)=10(x)2f(x) = 10(x)^2f(x)=10(x)2