Which of the following explains why a two part tariff causes customers who purchase few units to pay more per unit than customers who buy more units? a. customers who purchase few units pay a larger per unit price than those customers who buy more units b. the average lump sum fee is constant while the average per unit price decreases as the quantity purchased increases c. the average per unit price is constant while the average lump sum fee decreases as the quantity purchased increases d. customers who purchase few units pay a larger lump sum fee than those customers who buy more units