The chemical formulas of $A$,$B$,and $C$ are:
$A + Na_2CO_3 \to B + C$
$A \xrightarrow{CO_2} \text{Milky cloud}, C$

• A
  $A = Ca(OH)_2, B = NaOH, C = CaCO_3$
• B
  $A = NaOH, B = Ca(OH)_2, C = CaCO_3$
• C
  $A = NaOH, B = CaO, C = CaCO_3$
• D
  $A = CaO, B = Ca(OH)_2, C = NaOH$