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The correct answer is d. Tamoxifen is an antagonist because it prevents the binding of estrogen to the estrogen receptor by binding their first. It's binding produces no response and therefore limits the amount of cells produced, which limits the overall production of estrogen. In competitive inhibition, a molecule that resembles a substrate will bind to the active site, preventing the substrate itself from binding to that active site, thus preventing the enzyme from catalyzing the reaction.
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