If $$x$$, y, z are real numbers and not all zero and $$x + y + z = 0$$, then $$\frac{x^{2} + y^{2} + z^{2}}{(x - y)^{2} + (y - z)^{2} + (z - x)^{2}}$$

A

1

B

1/3

C

0

D

$$\frac{1}{2}$$