The value of $$\int_{0}^{1} \int_{0}^{1} \frac{dx dy} {\sqrt{ (1 -x^2)(1 - y^2)}}$$ is

A

$$\frac {\pi^2}{2}$$

B

$$\frac {\pi}{4}$$

C

$$\frac {\pi^2}{4}$$

D

$$\frac {\pi}{2}$$