Let f(x)=3x+5x−4f(x) = \frac{3x+5}{x-4}f(x)=x−43x+5. If f(f(x))=xf(f(x)) = xf(f(x))=x, which of the following must be true?
The function is a constant function.
The function is its own inverse.
The function has no domain restrictions.
The function is not surjective.