原式$$=\left( x-y+\frac{4xy}{x-y} \right)\left( x+y-\frac{4xy}{x+y} \right)$$  $$=\frac{{{x}^{2}}-2xy+{{y}^{2}}+4xy}{x-y}\cdot \frac{{{x}^{2}}+2xy+{{y}^{2}}-4xy}{x+y}$$  $$=\frac{{{(x+y)}^{2}}}{x-y}\cdot \frac{{{(x-y)}^{2}}}{x+y}$$  $$=(x+y)(x-y)$$  $$={{x}^{2}}-{{y}^{2}}$$．  故选$$\text{B}$$． 
化简分式$$\left( x-y+\frac{4xy}{x-y} \right)\left( x+y-\frac{4xy}{x+y} \right)$$的结果是．
Options:
a: $${{y}^{2}}-{{x}^{2}}$$ 
b: $${{x}^{2}}-{{y}^{2}}$$ 
c: $${{x}^{2}}-4{{y}^{2}}$$ 
d: $$4{{x}^{2}}-{{y}^{2}}$$ 