2HO HO_{2} H_{2}O_{2} H_{2}O
I gave my students this multiple choice question: Sodium bicarbonate (NaHCO_{3}) will neutralize stomach acid (HCl) in a double replacement reaction as follows: NaHCO_{3} + HCl -> NaCl + CO_{2} + \Box What is the product necessary to make this reaction complete?

Only one answer is correct among these 4 choices:
- 2HO
- HO_{2}
- H_{2}O_{2}
- H_{2}O

Could you tell me which one is correct?

H_{2}O