Use the curve $x^{2}-x y+y^{2}=1$.
Which of the following is equal to $\frac{d^{2} y}{d x^{2}} ?$
(A) $-\frac{6}{(2 y-x)^{3}}$
(B) $\frac{10 y^{2}-10 x^{2}-10 x y}{(2 y-x)^{3}}$
(C) $\frac{8 x^{2}-4 x y+8 y^{2}}{(x-2 y)^{3}}$
(D) $\frac{10 x^{2}+10 y^{2}}{(x-2 y)^{3}}$
(E) $\frac{2}{x}$