The correct answer is: C
Graph A demonstrates a stepped cost where a constant cost is charged while mileage stays within a set range and then increase to another constant cost as mileage slips into the next band.
Graph B illustrates a cost which is fixed for mileage up to a certain level and then increases at a constant cost per mile.
Graph D represents nil cost for the first quantity of mileage which then increases at a constant cost per mile once a set mileage has been reached. | 
