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Question 117·Hard·Text Structure and Purpose

The following text is from an essay by an urban designer about public spaces.

When cities install benches to make a plaza feel welcoming, they often find the benches stay empty. At lunchtime, people crowd stairs, planter rims, and the low edge of a fountain, balancing containers on their knees while the benches wait in the shade. People don't seek benches; they seek edges. Edges lend a back to lean against, a ledge to set a cup upon, a corner to share without feeling watched. That is why the same plaza, rearranged with a waist-high wall or a row of broad steps, fills overnight with readers, texters, and pairs in close conversation. The material is incidental—stone, wood, or brick will do. The geometry is not: a continuous line that makes a limit and, by doing so, makes a place.

Which choice best describes the function of the underlined sentence in the text as a whole?