Which of the following functions is its own inverse (i.e., f(f(x))=xf(f(x)) = xf(f(x))=x)?
f(x)=x2f(x) = x^2f(x)=x2
f(x)=1xf(x) = \frac{1}{x}f(x)=x1
f(x)=x+5f(x) = x + 5f(x)=x+5
f(x)=2xf(x) = 2xf(x)=2x