Select the graph that best matches the following description of cost behavior. For each graph, the vertical axis represents total dollars of cost, and the horizontal axis represents output units during the period. To encourage conserving water, the local water company uses a tier system for water service charge. For the first 18,000 gallons of water used, the per- gallon rate is the lowest. For the next 18,000 gallons of water used, the per- gallon rate is slightly higher. For any water used over 36,000 gallons, the per- gallon rate is much higher. A. C. B. D