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Choose the correct answer from the given four option.
$\tan^{-1}+\tan^{-1}\text{y}=\text{C}$ is the general solution of the differential equation:

1. $\frac{\text{d}\text{y}}{\text{d}\text{x}}=\frac{1+\text{y}^2}{1+\text{x}^2}$
   
2. $\frac{\text{d}\text{y}}{\text{d}\text{x}}=\frac{1+\text{x}^2}{1+\text{y}^2}$
   
3. $(1+\text{x}^2)\text{dy}+(1+\text{y}^2)\text{dx}=0$
   
4. $(1+\text{x}^2)\text{dx}+(1+\text{y}^2)\text{dy}=0$