The simplified value of ( 1 - 2xyx^2 + y^2 ) ÷ ( x^3 - y^3x - y - 3xy ) is?

Answer: Option B

$$\left(1-\frac{2xy}{x^2+y^2}\right)÷\left(\frac{x^3-y^3}{x-y}-3xy\right)$$
$$=\left(\frac{x^2+y^2-2xy}{x^2+y^2}\right)÷$$    $$\left(\frac{x^3-y^3-3xy\left(x-y\right)}{x-y}\right)$$
$$\begin{array}{rl}& =\frac{{\left(x-y\right)}^2}{x^2+y^2}÷\frac{{\left(x-y\right)}^3}{x-y}\\ & =\frac{{\left(x-y\right)}^2}{x^2+y^2}÷{\left(x-y\right)}^2\\ & =\frac{{\left(x-y\right)}^2}{x^2+y^2}\times \frac{1}{{\left(x-y\right)}^2}\\ & =\frac{1}{x^2+y^2}\end{array}$$