The Market for Breakfast Cereals Is Dominated by Four Producers

The market for breakfast cereals is dominated by four producers: Kellogg's, General Mills, Post Foods, and Quaker. The market contains hundreds of similar types of cereals, such as Fruit Loops, Cornflakes, and Rice Krispies, that are seen by buyers as different products. This situation violates the perfect competition assumption of:

A) many buyers and sellers.
B) an identical product.
C) ease of entry.
D) ease of exit.