Question: Which of the following is a correctly written chemical equation that demonstrates the conservation of mass?

  Options: 
 A. Mg + HCl -> H_{2} + MgCl_{2}
 B. H_{2}O + CO_{2} -> H_{2}CO_{3}
 C. KClO_{3} -> KCl + O_{2}
 D. H_{2} + O_{2} -> H_{2}O 
Answer: B