Answer: A. The rate of reaction depends on the concentration of the reactants. As the iron reacts with the dilute hydrochloric acid, the acid is gradually used up, so its concentration falls.
A lower acid concentration means fewer acid particles per unit volume, so there are fewer effective collisions per second between the acid and the iron. The rate therefore decreases as the reaction proceeds.
The temperature is held constant at 25 °C, so it does not change the trend. The reaction simply slows down until a reactant runs out — a standard collision-theory result.
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