The variable $x$ varies directly as the square of $y$, and $y$ varies directly as the cube of $z$. If $x$ equals $-16$ when $z$ equals 2, what is the value of $x$ when $z$ equals $\frac{1}{2}$?
(A)-1 (B)16 (C)-\frac{1}{256} (D)\frac{1}{16}
C