Mg + HCl -> H_{2} + MgCl_{2} H_{2}O + CO_{2} -> H_{2}CO_{3} KClO_{3} -> KCl + O_{2} H_{2} + O_{2} -> H_{2}O
Which of the following is a correctly written chemical equation that demonstrates the conservation of mass?

Options:
- Mg + HCl -> H_{2} + MgCl_{2}
- H_{2}O + CO_{2} -> H_{2}CO_{3}
- KClO_{3} -> KCl + O_{2}
- H_{2} + O_{2} -> H_{2}O

H_{2}O + CO_{2} -> H_{2}CO_{3}