(04.03) The graph shows the amount of money paid when purchasing bags of candy at the zoo: A graph is shown. The x-axis is labeled from 0 to 9 and the y-axis is labeled from 0 to 10. The four points shown on the graph are ordered pairs 0, 0 and 1, 2 and 2, 4 and 3, 6. These points are joined by a line. The label on the x-axis is Bags of candy. The title on the y-axis is Total cost in dollars.

What is the constant of proportionality?

a. one-half
b. three-fourths
c. 1
d. 2