Selection is a method related to which of the following?

If $(2, 0)$ is the vertex and the $y$-axis is the directrix of a parabola,then its focus is

$4 \tan^{-1} \frac{1}{5} - \tan^{-1} \frac{1}{239}$ is equal to

If $\frac{d}{d x}\left[(x+1)\left(x^2+1\right)\left(x^4+1\right)\left(x^8+1\right)\right] = \left(15 x^p-16 x^q+1\right)(x-1)^{-2}$,then $(p, q)$ is equal to

What is $X$ in the following reaction?
$MgCl_2 + 2H_2O \to X + 2HCl + H_2$

Charges $+q$ and $-q$ are placed at points $A$ and $B$ respectively,which are at a distance $2L$ apart. $C$ is the midpoint between $A$ and $B$. The work done in moving a charge $+Q$ along the semicircle $CRD$ is: