At a factory, the cost of making different numbers of toothbrushes is shown in the table below. COST OF TOOTHBRUSHES Number of 3 Toothbrushes Cost (dollars) 9 12 $4.50 $9.00 $13.50 $18.00 A linear function models the cost based on the number of toothbrushes made. Which statement about the rate of change of this function is true? A