According to the law of conservation of mass, if an atom of element A has a mass of 2 units, while an atom of element B
has a mass of 5 units, what would be the mass of the
compound A3B2 ?

Combustion is the act or process of burning. therefore, i believe the answer is  a: burning wood. 

I'm sorry i cant you if you don't list the elements : (

Answer is: hydrogen is being oxidized.chemical reaction: co₂ + h₂ → co + h₂o.oxidation half reaction: h₂ → 2h⁺ + 2e⁻.reduction half reaction: c⁺⁴ + 2e⁻ → c⁺².oxidation is increase of oxidation number, hydrogen change oxidation number from neutral charge (in molecule of hydrogen) to oxidation number +1 (in water).