Find $\frac {dy}{dx}$ in the following: $y = cos^{-1} \left(\frac{1 - x^2}{1 + x^2}\right), 0 < x < 1$

Question

$\begin{array}{1 1} \frac{2}{1+x^2} \\ \frac{-2}{1+x^2} \\ \frac{1-x^2}{1+x^2} \\ \frac{1+x^2}{1-x^2} \end{array}$