Dynamic equilibria

A new experiment, for which the balanced chemical equation is unknown, was performed in the lab.
aA(aq)+bB(aq)dD(aq)aA(aq)+bB(aq)\rightleftarrows dD(aq)

The concentrations of the different species (A, B and D) were monitored and plotted as a function of time, as shown below. Which of the following statements concerning this reaction is true?
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