For each of the following explain the degree of market power the seller has, and whether the market power is due to limited competitors, product differentiation, or both. 

a. The Toyota Corolla, the bestselling passenger vehicle in the world in 2017.

b. Heartland Mills, a grower of wheat in Wichita County, Kansas.

c. Archer Daniels Midland (ADM), the largest producer of Lysine, a standardized amino acid used by farmers as a feed additive. ADM competes with a few large corporations in Europe and Japan. 

d. McDonald’s, the most popular fast food restaurant in the United States.