Final answer:
An antagonist is a chemical that slows down a chemical reaction by inhibiting or blocking the action of a catalyst or reactant.
Step-by-step explanation:
An antagonist refers to a chemical that slows down a chemical reaction. It works by inhibiting or blocking the action of a catalyst or reactant, reducing the rate at which the reaction occurs.
For example, in the context of enzyme-catalyzed reactions, an antagonist can bind to the active site of the enzyme, preventing the substrate from binding and impeding the reaction.
This is different from a chemical that is added to a reaction (option C) or one that speeds up a reaction (option D). Option B refers to a chemical that is completely consumed or transformed in a reaction, which is not the case for an antagonist.
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