Complete the following reactions:
$(a)$ $4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \to 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$
$(b)$ $2Na_2CrO_4 + 2H^+ \to Na_2Cr_2O_7 + 2Na^+ + H_2O$
Identify the species $X$ in both reactions.

• A
  $X = Na_2CO_3$ in $(a)$ and $X = 2H^+$ in $(b)$
• B
  $X = NaOH$ in $(a)$ and $X = H_2O$ in $(b)$
• C
  $X = Na_2O$ in $(a)$ and $X = OH^-$ in $(b)$
• D
  $X = NaHCO_3$ in $(a)$ and $X = H_2O$ in $(b)$