$A$ metal $(A)$ on heating in nitrogen gas gives compound $(B)$. $(B)$ on treatment with $H_{2}O$ gives a colourless gas which when passed through $CuSO_{4}$ solution gives a dark blue-violet coloured solution. $(A)$ and $(B)$ respectively,are

• A
  $Mg$ and $Mg_{3}N_{2}$
• B
  $Na$ and $NaNO_{3}$
• C
  $Mg$ and $Mg(NO_{3})_{2}$
• D
  $Na$ and $Na_{3}N$