(D) The $s$-block elements have their last electron entering the $s$-orbital.Atomic number $3$ corresponds to $Li$ $(1s^2 2s^1)$,which is an $s$-block

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(D) The $s$-block elements have their last electron entering the $s$-orbital.Atomic number $3$ corresponds to $Li$ $(1s^2 2s^1)$,which is an $s$-block element.Atomic number $12$ corresponds to $Mg$ $([Ne] 3s^2)$,which is an $s$-block element.Therefore,the pair $(3, 12)$ represents $s$-block elements.