If the direction cosines of two lines are such that $l+m+n=0; l^2 +m^2-n^2=0$ then the angle between them is :

Question

\[\begin {array} {1 1} (a)\;\pi & \quad (b)\;\pi/3 \\ (c)\;\pi/4 & \quad (d)\;\pi/6 \end {array}\]