Answer: D. Heat is treated as a reactant or a product
Step-by-step explanation:
The effect of heat on a reaction be determined when heat is treated as a reactant or a product.
In this case, if heat is the reactant, it'll be the substance that will undergo a change during the reaction. It should be noted that reactant is present at the beginning of a chemical reaction.
On the other hand, as the product, heat will be present when the chemical reaction is completed. Therefore, the correct option is D.