If $$x= \frac{\sqrt{3} + 1}{\sqrt{3} - 1}, xy = 1,$$ the value of $$\left(\frac{x - y}{x + y}\right)^2$$ is

A

$$\frac {4}{3}$$

B

$$\frac{2}{\sqrt 3}$$

C

$$\frac {\sqrt 3}{2}$$

D

$$\frac {3}{4}$$