If $$v = (x^2 + y^2 + z^2)^{\frac {-1}{2}}$$, then $$\frac{\partial^2 v}{\partial x^2} + \frac{\partial^2 v}{\partial y^2} + \frac{\partial^2 v}{\partial z^2}$$ is

A

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

B

$$-1$$

C

$$0$$

D

$$1$$