The number of straight lines that are equally inclined to the three-dimensional coordinate axes is

The direction cosines of the line passing through $P(2, 3, -1)$ and the origin are

$\text{Assertion (A)}$: The direction ratios of line $L_1$ are $2, 5, 7$ and those of line $L_2$ are $\frac{4}{\sqrt{19}}, \frac{10}{\sqrt{19}}, \frac{14}{\sqrt{19}}$. The lines $L_1, L_2$ are parallel.
$\text{Reason (R)}$: The direction ratios of a line $L_1$ are $a_1, b_1, c_1$ and those of another line $L_2$ are $a_2, b_2, c_2$. The lines $L_1$ and $L_2$ are parallel if $a_1 a_2+b_1 b_2+c_1 c_2=0$.
The correct option among the following is

If $l_1, m_1, n_1$ and $l_2, m_2, n_2$ are direction cosines of $OA$ and $OB$ such that $\angle AOB = \theta$,where $O$ is the origin,then the direction cosines of the internal angular bisector of $\angle AOB$ are

If the direction ratios $(l, m, n)$ of two lines satisfy the equations $l+m+n=0$ and $mn-2ln+lm=0$, then the angle between the lines is

The angle between the straight lines,whose direction cosines are given by the equations $2l + 2m - n = 0$ and $mn + nl + lm = 0$,is: