The angle between the lines whose direction cosines satisfy the equations $\;l+m+n=0\;$ and $\;l^{2}=m^{2}+n^{2}\;$ is :

Question

$(a)\;\large\frac{\pi}{6} \qquad(b)\;\large\frac{\pi}{2}\qquad(c)\;\large\frac{\pi}{3}\qquad(d)\;\large\frac{\pi}{4}$