Which of the following functions is onto ?

$f: R→R, f(x) = x^2 $ $f: R→R, f(x) = x^2 +1$ $f: R→R, f(x) = x^2-1 $ $f: R→R, f(x) = 2x+1 $

$f: R→R, f(x) = 2x+1 $