The lengths of the sides of a triangle $\alpha -\beta , \alpha+\beta$ and $\sqrt {3 \alpha^2+\beta^2},(\alpha > \beta > 0)$. Its largest angle is

Question

$\begin{array}{1 1} \large\frac{3 \pi}{4} \\ \large\frac{\pi}{2} \\\large\frac{2 \pi}{3} \\\large\frac{5 \pi}{6} \end{array}$