The graph shows the volume of a gaseous product formed during two trials of a reaction. A different concentration of reactant was used during each trial, whereas the other factors were kept constant.
Which of the following statements explains which trial has a lower concentration of the reactant?

A. Trial 1, because the average rate of the reaction is lower.

B. Trial 1, because this reaction lasted for a longer duration than Trial 2.

C. Trial 2, because this reaction was initially fast and later slowed down.

D. Trial 2, because the volume of product formed per unit time was higher.