$\tan^{-1}x+\tan^{-1}y=c$ is the general solution of the differential equation:

Question

$$\begin{array}{1 1}(A)\;\frac{dy}{dx}=\frac{1+y^2}{1+x^2} & (B)\;\frac{dy}{dx}=\frac{1+x^2}{1+y^2}\\(C)\;(1+x^2)dy+(1+y^2)dx=0 & (D)\;(1+x^2)dx+(1+y^2)dy=0\end{array}$$