\textbf{78--} If element $X$ and element $_{28}$Ni are in the same period and in the same group, and are constructed as nuclear reactor elements, the \textbf{electronic configuration of its cation in the compound .................. is of the form ....................}
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$[_{18}Ar]3d^54s^2\ ,\ X_rO_r\ (1) \qquad\qquad [_{18}Ar]3d^54s^2\ ,\ XCl_r\ (2)$
$[_{18}Ar]3d^4\ ,\ XCl_r\ (3) \qquad\qquad [_{18}Ar]3d^4\ ,\ X_rO_r\ (4)$
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