When methane (CH4) combines with oxygen gas (O2), water vapor (H2O), and carbon dioxide (CO2) are produced. Explain how mass is conserved in this reaction. Use a model to support your explanation.

A. Mass is not conserved.
B. Mass is conserved because the total number of atoms before and after the reaction is the same.
C. Mass is conserved because the reactants and products have the same density.
D. Mass is conserved because the reaction occurs in a closed system, preventing the escape of mass.