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?
What is the constant of proportionality?

one-half
three-fourths
1
2