PLEASE HELP! 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.

(I've attached the graph.)

Which of the following statements explains which trial has a lower concentration of the reactant?

a. Trial X, because the final volume of product formed is lower than Trial Y.
b. Trial X, because this reaction was initially fast and later stopped completely.
c. Trial Y, because the reaction was initially slow and later stopped completely.
d. Trial Y, because the volume of product formed per unit time is lower than Trial X.