The change in concentration of reactants or products per unit time is called rate of reaction. The rate of reaction:

Explanation

The rate of reaction typically decreases as the reaction progresses due to the depletion of reactants and the accumulation of products. This results in fewer effective collisions between reactant molecules, thus slowing the reaction rate. Option A is incorrect because the rate usually does not increase without external influences. Option C is incorrect because the rate of reaction does not remain constant due to changing concentrations. Option D is misleading without additional context, as natural progression generally leads to a decrease in rate.