$Mg(NO_3)_2 \cdot XH_2O$ and $Ba(NO_3)_2 \cdot YH_2O$ represent the formulas of the crystalline forms of nitrate salts. The sum of $X$ and $Y$ is $.........$.

• A
  $6$
• B
  $5$
• C
  $4$
• D
  $3$