单项选择题

Historians often write of world history in terms of the development of civilizations defined by a characteristic empire. The regions of Mesopotamia, Egypt (the Nile Valley), and the Indus Valley are three rich areas for studying how people and ideas come together to create civilizations and empires.
Imagine three spaces that are sparsely populated, yet well watered and fertile, in a time before written history. Two are river valleys, another lies between two rivers forming a rich plain. Imagine that humans settle in these regions and domesticate plants and animals. The domestication made possible by these river territories and the success of that domestication — farming and grazing — lure increasingly greater human and animal migration to these spaces. As these populations increase, so do their needs. These needs give rise to the social and political economic formations that characterize the ancient urban spaces and states of Mesopotamia, the Indus and Nile valleys.
The initial formation of Mesopotamian, Egyptian, and Indus Valley civilizations is based on the movement of peoples into the river valleys and plains. The transformation of these valleys and plains into places capable of physically nurturing the various peoples who moved into them was one of the first acts of cultural innovation and exchange. The use of these valleys’ soil and water was signs of innovation and exchange. For the Mesopotamians, the key to making the land fertile was the technology of irrigation. Egypt and the Nile Valley civilizations were defined by the rich alluvial soils that annual floods deposited along the Nile banks and in the delta and flood plains. The use of water and the timing of flood seasons gave rise to a number of technological innovations, such as the calendar. These cultural and technological innovations also guaranteed the growth of large populations and increased the possibility that some of those populations would be located in central urban centers.
These societies’ agricultural and ecological technologies drew immigrants and travelers. Some of these people entered the areas peaceably. Others used force to maintain or expand geographic and cultural spaces, indicating imperial activity. An interesting pattern emerging here in some urban centers was constructed to protect against invading forces, and seen in the walled settlements of the Indus Valley and early Mesopotamia. However, as much as these walled settlements repelled invaders, they also attracted them. The river valleys and the plains, and their agricultural richness, supported the formation of cities. The cities became emblems of their respective empires and either allowed for the extension of the empire or resisted the threats of other powers.
The historical activities of the Indus Valley, Mesopotamia, and Egypt indicate that various peoples moved in and out, contested the regions’ spaces, and sought to control other peoples, their goods and their resources. Richness is understood as the population’s ability to produce goods and services in quantity not just agriculture, but skills such as metalworking, pottery, or commerce. Thus, richness in population meant surpluses allowed the cities and the areas they controlled to support a ruling and administrative class, and maybe an array. Frequently, product surpluses were exchanged, providing wealth for the area and drawing other peoples to it. The Indus Valley, Mesopotamia, and Egypt all experienced the results of a rich and productive population.
New language patterns, such as the early substitution of the Akkadian tongue for Sumerian, demonstrate the innovations encouraged by movement and exchange. Diplomatic exchange as well as military struggle resolved conflict over the empires’ boundaries and areas of control. Marriage was a highly visible form of diplomacy and amounted to an exchange between ruling families that linked them politically and economically. These arrangements often resulted in the cessation of hostilities, greater regional stability, and greater economic exchange. Marriages across the ruling classes of these societies offer one way to conceptualize the world. Political marriages and royal hostages both provided for the sharing of culture across religious and ethnic divisions and differences that may well have contributed to humankind’s history.
The establishment of empires, and the civilizations they represented, was not the creation of discrete imperial space so much as a way of ordering interaction between possible discrete spaces. The structures of these civilizations — these empires, states, cities — encouraged the interaction and the flow of goods, people, and ideas. On the contrary, they encouraged it. That encouragement resulted in the earliest formations of what has been called the Afro-Eurasian Old World — the interaction between the Indus, Mesopotamian, and Nile river systems. How should we understand the richness of these three civilizations according to the passage