A4 Which of the following statements is true for the rational function $f$ defined by $f(x)=x^{-1}-\frac{1}{x^{3}}$?
(A) The function has exactly one zero at $x=1$.
(B) The function has a horizontal asymptote $y=-1$.
(C) The function has no poles.
(D) The domain of the function is $\mathbb{R}-\{0\}$.
(E) The function has no asymptotes.
A4. We transform the equation of the rational function into the form $f(x)=\frac{x^{2}-1}{x^{3}}$ and determine that the correct answer is D.