In between the metals A and B, both form oxide but B also forms nitride, when both are heated in air. A and B are respectively

Lithium easily reacts with atmospheric N2 and forms lithium nitride "Li3N2"Where as remaining IA group elements reacts with nitrogen at high temperature onlySo,"B" is "Li" as it reacts with atmospheric "N2" under ordinary conditionBoth A[K] & B[Li] forms oxides with atom spheric air [Oxygen]Note :