In which of the following pairs,electron gain enthalpies of constituent elements are nearly the same or identical?
$A$. $Rb$ and $Cs$
$B$. $Na$ and $K$
$C$. $Ar$ and $Kr$
$D$. $I$ and $At$
Choose the correct answer from the options given below.

The formation of the oxide ion,$O^{2-}_{(g)}$,from oxygen atom requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^- \to O^{-}_{(g)} ; \Delta_f H^{\Theta} = -141 \ kJ \ mol^{-1}$
$O^{-}_{(g)} + e^- \to O^{2-}_{(g)} ; \Delta_f H^{\Theta} = +780 \ kJ \ mol^{-1}$
Thus,the process of formation of $O^{2-}$ in gas phase is unfavourable even though $O^{2-}$ is isoelectronic with neon. It is due to the fact that,

Which elements have the most positive and most negative electron gain enthalpy,respectively?

Among the options, the element with the highest electron gain enthalpy is

Column-$I$ shows the electronic configuration of some elements and Column-$II$ shows the values of electron gain enthalpy. Match these values correctly. Match the electronic configuration with the electron gain enthalpy.

Column-$I$ Electronic Configuration	Column-$II$ Electron Gain Enthalpy / $kJ \ mol^{-1}$
$(A) \ 1s^2 2s^2 2p^6$	$(1) -53$
$(B) \ 1s^2 2s^2 2p^6 3s^1$	$(2) -328$
$(C) \ 1s^2 2s^2 2p^5$	$(3) -141$
$(D) \ 1s^2 2s^2 2p^4$	$(4) +53$

The electron gain enthalpy (with negative sign) of fluorine is less than that of chlorine due to: