A student made a graph to show the chemical equilibrium position of a reaction. The student forgot to label the y-axis of the graph.

A graph is shown with two graph lines. One graph line starts at a higher position on the y axis and slopes downwards towards the right. The other graph line starts at a lower position on the y axis and slopes upwards towards the right. The two graph lines intersect and continue as separate lines which gradually become straight and parallel to the x axis. A vertical line is shown at a point where the two graph lines finally became parallel to the x axis. This vertical line is labeled equilibrium. The title on the x axis is Time and an arrow pointing towards the right is shown above Time. The title on the y axis is left blank.

What best explains the label that the student should use on the y-axis?

Reaction rate, as the rate of forward and back reaction become equal at equilibrium.
Reaction rate, as the rate of forward reaction increases and rate of backward reaction decreases with time.
Concentration, as the concentration of products decreases and that of reactants increase over time.
Concentration, as the concentrations of reactants and products remain constant after equilibrium is reached.