The graph shows the amount of money paid when purchasing bags of peanuts at the zoo: A graph is shown. The x-axis is labeled from 0 to 9. The y-axis is labeled from 0 to 13. The four points shown on the graph are ordered pairs 0, 0 and 1, 4 and 2, 8 and 3, 12. These points are joined by a line. The label on the x-axis is Bags of peanuts. The title on the y-axis is Total cost in dollars.

Write an equation to represent the relationship between the total cost (y) and the number of bags of peanuts (x).

y = 4x
y = one-fourth x
y = 3x
y = one-third x