A graph titled Cost of Erasers. The x-axis shows the number of erasers, numbered 1 to 9, and the y-axis shows the total cost in dollars, numbered 0.5 to 4.5. Blue diamonds appear at points (1, 0.5), (2, 1), (3, 1.5), (4, 2), (5, 2.5), (6, 3), (7, 3.5), (8, 4), (9, 4.5). The graph shows the relationship between the total cost and the number of erasers bought at the student store. Which statement is true?