The effect of addition of helium gas to the following reaction in equilibrium state, is :

$$\mathrm{PCl}_{5}(\mathrm{g}) \rightleftharpoons \mathrm{PCl}_{3}(\mathrm{g})+\mathrm{Cl}_{2}(\mathrm{g})$$

Which one of the following sets of ions represents a collection of isoelectronic species?

(Given : Atomic Number : $$\mathrm{F:9,Cl:17,Na=11,Mg=12,Al=13,K=19,Ca=20,Sc=21}$$)

For electron gain enthalpies of the elements denoted as $$\Delta_{\mathrm{eg}} \mathrm{H}$$, the incorrect option is :

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R)

Assertion (A) : An aqueous solution of $$\mathrm{KOH}$$ when used for volumetric analysis, its concentration should be checked before the use.

Reason (R) : On aging, $$\mathrm{KOH}$$ solution absorbs atmospheric $$\mathrm{CO}_{2}$$.

In the light of the above statements, choose the correct answer from the options given below :