Hello!
The statement that best proves that the law of conservation is obeyed during this process is that The total mass of oxygen and carbon is equal to the total mass of carbon dioxide.
The chemical equation for the combustion reaction of coal in the carbon cycle is the following one:
C(s) + O₂(g) → CO₂(g)
From this simple chemical equation, we can conclude that if the Law of Conservation of Matter is obeyed in this reaction, the sum of the initial masses of Carbon and Oxygen before the reaction will be equal to the total amount of Carbon Dioxide formed by it, because they sum up to produce Carbon Dioxide.
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