$Zn$ and $Cd$ metals do not show variable valency because

There are two coils $A$ and $B$ as shown in the figure. No current flows in $B$ if $A$ is at rest. Now,the coil $A$ is made to rotate about a vertical axis. At the shown instant $(t = 0)$,what will be the direction of the current in coil $A$ if the induced current in coil $B$ is counterclockwise?

Identify the incorrect pair from the following:

The electronic configurations of $Cr^{2+}, Mn^{2+}, Fe^{2+},$ and $Ni^{2+}$ are $3d^4, 3d^5, 3d^6,$ and $3d^8$ respectively. Which one of the following aqua complexes will exhibit the minimum paramagnetic behavior?

An elementary particle of mass $m$ and charge $+e$ is projected with velocity $v$ at a much more massive particle of charge $Ze$,where $Z > 0$. What is the closest possible approach of the incident particle?

If the freezing point of an aqueous solution of salt $MX_4$ $(\alpha = 90\%)$ is $-3\,^{\circ}C$,then its boiling point will be ......... $^{\circ}C$. (Assume it is $100\%$ ionized at boiling temperature and $k_f = 1.86\,K\,kg\,mol^{-1}, k_b = 0.52\,K\,kg\,mol^{-1}$)