Here's a problem to solve: Which of the following is a correctly written chemical equation that demonstrates the conservation of mass?

Among the 4 following options, which is the correct answer?

- 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
 