A、$$G(x,y) = \frac{1}{2\pi\sigma} e^{-\frac{x^2+y^2}{\sigma}}$$
B、$$G(x,y) = \frac{1}{2\pi\sigma^2} e^{-\frac{x^2+y^2}{2\sigma^2}}$$
C、$$G(x,y) = \frac{1}{\sigma\sqrt{2\pi}} e^{-\frac{x^2}{2\sigma^2}}$$
D、$$G(x,y) = \frac{1}{2\pi} e^{-\frac{x^2+y^2}{2}}$$