Paragraph 6 indicates which of the following about the application of the Park and Burgess model to modern North American cities? A. It is especially useful for those cities that have been used as models for international development. B. It remains useful in explaining the development of some urban areas but not all cities. C. It can be applied equally well to cities with commercial centers and those with palaces and cathedrals at their center. D. It is less applicable to modern cities because of changes in patterns of immigration.