Two sociologists, Robert Park and Ernest Burgess, developed the "concentric-zone model" of how cities use land and grow. A. The model was developed to explain how the city of Chicago was developing around centrally located transportation and communication systems. B. The model arose out of concern for the quality of life in the rapidly growing cities of early twentieth-century America. C. The founders of the model did not believe in formal city planning and instead advocated growth through the expansion of so-called "natural areas." D. According to the model, a group new to the city tends to live together near the center and over time moves to outer areas that are more diverse ethnically and occupationally. E. The model is applicable to cities that grow by attracting large numbers of workers to centrally located businesses. F. The model predicts that eventually the inner city becomes so crowded that its residents move to new satellite cities outs de the city limits.