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? (5 points)

a
Reaction rate, as the rate of forward and back reaction become equal at equilibrium.

b
Reaction rate, as the rate of forward reaction increases and rate of backward reaction decreases with time.

c
Concentration, as the concentration of products decreases and that of reactants increase over time.

d
Concentration, as the concentrations of reactants and products remain constant after equilibrium is reached.