Park and Burgesss model has come to be known as the "concentric－zone model"（represented by

Park and Burgesss model has come to be known as the "concentric-zone model"(represented by the figure). Because the model was originally based on studies of Chicago, its center is labeled "Loop," the term commonly applied to that citys central commercial zone. Surrounding the central zone is a "zone in transition," an area that is being invaded by business and light manufacturing. The third zone is inhabited by workers who do not want to live in the factory or business district but at the same time need to live reasonably close to where they work. The fourth or residential zone consists of upscale apartment buildings and single-family homes. And the outermost ring, outside the city limits, is the suburban or commuters zone; its residents live within a 30-to 60-minute ride of the central business district.

The word "they" in the passage refers

A．Chicago-school sociologists

B．new groups of immigrants

C．separate areas

D．inner-city zones

请帮忙给出正确答案和分析，谢谢！