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

A. [tex]3x\left(\frac{\pi}{3}\right)[/tex]
B. [tex]\left(\frac{1}{3}x\right)(3x)[/tex]
C. [tex]\frac{1}{3}(3x)[/tex]
D. [tex]\frac{1}{3}\left(\frac{1}{3}x\right)[/tex]