The angle between the lines whose $d.c.$ are given by the equations $3l+m+5n=0$ and $6mn-2nl+5lm=0$ is?

Question

$\begin{array}{1 1} (a)\:\:0\:\:\:\:\qquad\:\:(b)\:\:\large\frac{\pi}{2}\:\:\:\:\qquad\:\:(c)\:\:cos^{-1}\large\frac{1}{3}\:\:\:\:\qquad\:\:(d)\:\:cos^{-1}\large\frac{1}{6} \end{array}$