$A$ metal $M$ reacts with $N_2$ to form a compound $A$ $(M_3N)$. Heating $A$ at high temperature gives back $M$,and passing $A$ through $H_2O$ produces gas $B$. Gas $B$ turns $CuSO_4$ solution into a deep blue color. What are $A$ and $B$?

• A
  $Mg$ and $NH_3$
• B
  $Na$ and $NH_3$
• C
  $Li$ and $NH_3$
• D
  $Al$ and $NH_3$