Which of the following functions f:R→Rf: \mathbb{R} \to \mathbb{R}f:R→R is a bijection?
f(x)=x2f(x) = x^2f(x)=x2
f(x)=x3+xf(x) = x^3 + xf(x)=x3+x
f(x)=sin(x)f(x) = \sin(x)f(x)=sin(x)
f(x)=exf(x) = e^xf(x)=ex