A metal $\mathrm{M}$reacts with nitrogen gas to afford ${\text{M}}_3\text{N}$, ${\text{M}}_3\text{N}$ on heating at high temperature gives back $\mathrm{M}$and on reaction with water produces a gas $\mathrm{B}$. Gas $\mathrm{B}$reacts with an aqueous solution of ${\mathrm{CuSO}}_4$ to form a deep blue compound. $\mathrm{M} \mathrm{and} \mathrm{B}$ respectively are

• Correct answer
  A

  $\mathrm{Li}$ and ${\mathrm{NH}}_3$.
• B

  $\mathrm{Ba}$ and ${\mathrm{NH}}_{3.}$
• C

  $\mathrm{Na}$ and ${\mathrm{N}}_2.$
• D

  $\mathrm{Al}$ and ${\mathrm{N}}_{2.}$

Hence, the correct answer is option: 'A'.