Hello!
The statement that best explains whether the mass is conserved or not during photosynthesis is that It is conserved because the total number of O atoms on each side is 18.
This comes from the Law of Conservation of Mass. It was discovered by Antoine Lavoisier, who is often called "the father of Chemistry". It states that matter is neither created nor destroyed in chemical reactions and that it only changes. In Photosynthesis, the chemical reactions transform the Oxygen in carbon dioxide and water to sugars and molecular oxygen.
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