Which function represents exponential decay?
f(x)=0.5xf(x) = 0.5^xf(x)=0.5x
f(x)=1.5xf(x) = 1.5^xf(x)=1.5x
f(x)=exf(x) = e^xf(x)=ex
f(x)=2xf(x) = 2^xf(x)=2x