Which equation shows that $z$ varies directly with the square of $x$ and inversely with the cube of $y ?$
$$
\begin{array}{llll}{\text { A. } z=\frac{x^{2}}{y^{3}}} & {\text { B. } z=\frac{x^{3}}{y^{2}}} & {\text { C. } z=\frac{y^{2}}{x^{3}}} & {\text { D. } z=\frac{y^{3}}{x^{2}}}\end{array}
$$