Gabriel is accelerating on his bike
The answer is going to be (a) - oxygen. this is because, for this reaction, charcoal undergoes a combustion reaction, using the oxygen from the surrounding atmosphere to burn to heat, light and CO2.
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I don't understand your question
The reaction CO₂(g) → C(s) +O₂(g) is a decomposition reaction is true
Explanation
A Decomposition reaction is a reaction in which involve breaking down a compound into two or more simpler substances.
The reaction above is a decomposition reaction since CO₂ molecule is broken down into C (an element) and O₂ (a molecule).