Final answer:
The Burgess Concentric Zone Model explains the distribution of different social groups within urban areas, the Hoyt Sector Model involves wedge-shaped zones flanking industrial corridors, and the Ullman/Harris Multiple Nuclei Model proposes more than one center in a city.
Step-by-step explanation:
The first model to explain the distribution of different social groups within urban areas is the Burgess Concentric Zone Model. This model views a city as a series of concentric circular areas, expanding outward from the center of the city. It describes different zones, such as the business district in Zone A, formerly wealthy homes and cheap apartments in Zone B, working class homes in Zone C, and wealthy homes in Zone D. This model suggests that as the city grows, activities expand outwards in a wedge from the center.
The Hoyt Sector Model is another model of urban structure. It suggests that cities generally have wedge-shaped zones cutting across the concentric rings found in the concentric zone model. The CBD remains in the middle, but working-class homes are commonly found flanking industrial corridors that stretch away from the CBD along transportation routes.
The Ullman/Harris Multiple Nuclei Model is a model that suggests there is more than one center around which activities revolve in a city. It features various zones, including industrial districts, working-class housing, upper middle-class housing, elite housing, light industrial & wholesale areas, commuter housing, and exurban industrial areas.