Park and Burgess based their model of urban growth on the concept of "natural areas"—that

Park and Burgess based their model of urban growth on the concept of "natural areas"—that is, areas such as occupational suburbs or residential enclaves in which the population is relatively homogeneous and land is used in similar ways without deliberate planning. Park and Burgess saw urban expansion as occurring through a series of "invasions" of successive zones or areas surrounding the center of the city. For example, people from rural areas and other societies "invaded" areas where housing was inexpensive. Those areas tended to be close to the places where they worked. In turn, people who could afford better housing and the cost of commuting "invaded" areas farther from the business district.

According to paragraph 4, why is the term "Loop" used in the concentric-zone model？

A．It indicates the many connections between each of the zones in the model.

B．It indicates that zones are often in transition and frequently changing.

C．It reflects the fact that the model was created with the city of Chicago in mind.

D．It emphasizes the fact that populations often returned to zones in which they used to live.

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