If $f(x)=3 x$ and $g(x)=\frac{1}{3} x$, which expression could be used to verify that $g(x)$ is the inverse of $f(x) ?$
$3 x\left(\frac{x}{3}\right)$
$\left(\frac{1}{3} x\right)(3 x)$
$\frac{1}{3}(3 x)$
$\frac{1}{3}\left(\frac{1}{3} x\right)$