Determine the market structure for the following cases and explain your reasoning.

a) The place where you live is like many other places, you and your friends have many choices about where to go to get a haircut. The price you pay for a basic haircut probably ranges from a few dollars at a discount establishment to many dollars at an upscale salon.

b) The four largest breakfast cereal companies (Kellogg, General Mills, Post, and Quaker) were producing over 86 percent of the total amount of breakfast cereals in the United States. These cereal producers spend a lot on advertising and use advertising as a way to compete with one another.

c) Beginning in the 1930s and throughout most of the 20th century, the De Beers company, based in Switzerland and South Africa, controlled most of the world's diamond supply. Control of the supply of diamonds enabled De Beers to restrict the number of diamonds offered for sale and sell them at higher prices than would exist under competition.