$A$ metal $M$ reacts with nitrogen gas to afford $M_3N$. $M_3N$ on heating at high temperature gives back $M$ and on reaction with water produces a gas $B$. Gas $B$ reacts with aqueous solution of $CuSO_4$ to form a deep blue compound. $M$ and $B$ respectively are

• A
  $Li$ and $NH_3$
• B
  $Ba$ and $N_2$
• C
  $Na$ and $NH_3$
• D
  $Al$ and $N_2$