Read the passage and answer the question. The Emergence of the First Cities For most of human history, people lived in small, mobile groups that subsisted by hunting animals and gathering wild plants. The appearance of the first cities, beginning around 5,500 years ago in the river valleys of the Near East, therefore represents one of the most profound transformations in the human story. Yet the question of why cities arose has no simple answer, and scholars continue to weigh several competing explanations. The most widely cited factor is the surplus made possible by agriculture. Once farmers in fertile regions such as Mesopotamia could grow more grain than they immediately needed, that surplus could feed people who did not farm at all — potters, weavers, priests, and administrators. Specialization of labor followed, and with it the dense, interdependent populations that define urban life. Critics of this view point out, however, that surplus alone does not compel people to gather in cities; many agricultural societies produced surpluses for centuries without urbanizing. A second explanation emphasizes the role of trade. Cities frequently arose at points where goods naturally changed hands — river junctions, mountain passes, and coastlines. Such locations attracted merchants and the artisans who served them, and the wealth generated by exchange could support large settlements even in areas with limited farmland. A third view stresses administration and defense: as populations grew, the need to coordinate irrigation, store grain, resolve disputes, and protect against raiders may have drawn people together under centralized authority, often expressed through monumental temples and walls. These explanations are not mutually exclusive. Most archaeologists now suspect that the earliest cities emerged from a combination of pressures that varied from place to place. What seems clear is that once established, cities became engines of innovation, concentrating people and ideas in a way that accelerated the development of writing, mathematics, and law. ────────── Why does the author mention "monumental temples and walls" in paragraph 3?