Walgreens is based in Chicago, Illinois, and has developed a large customer base, especially in larger cities. It has strong relationships with its suppliers and an innovative supply chain, enabling it to reduce out-of-stock inventory, especially with fast-moving items that are purchased frequently. These capabilities have enabled Walgreens to maintain its market position and have a strong competitive position since they are not easily copied or duplicated by other firms. Walgreens enjoys a _______ competitive advantage. a. cost b. sustainable c. niche d. product/service differentiation