The amount of oxygen in the reactants is 4 atoms. In the products, the oxygen is distributed to water (H2O) and O2 gas. Which coefficients would be placed in front of water and oxygen?

Each H₂O molecule contains 1 oxygen atom, and each O₂ molecule contains 2. With coefficients 2 for water and 1 for oxygen gas, the products contain 2 × 1 + 1 × 2 = 4 oxygen atoms, matching the reactants. So the correct choice is 2 in front of water and 1 in front of oxygen.

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