A metal (A) on heating in nitrogen gas gives compound B. B on treatment with H₂O gives a colourless gas which when passed through CuSO₄ solution gives a dark blue-violet coloured solution. A and B respectively, are :
(1) Mg and Mg₃N₂
(2) Na and NaNO₃
(3) Mg and Mg(NO₃)₂
(4) Na and Na₃N
