A function fff has the property f(x+y)=f(x)f(y)f(x+y) = f(x)f(y)f(x+y)=f(x)f(y). Which of these could be f(x)f(x)f(x)?
f(x)=2xf(x) = 2xf(x)=2x
f(x)=x2f(x) = x^2f(x)=x2
f(x)=3xf(x) = 3^xf(x)=3x
f(x)=sin(x)f(x) = \sin(x)f(x)=sin(x)