What economic activity was at the center of each of these civilizations?

A civilization is a complex society that creates agricultural surpluses, allowing for specialized labor, social hierarchy, and the establishment of cities. Developments such as writing, complex religious systems, monumental architecture, and centralized political power have been suggested as identifying markers of civilization, as well. When we see these changes occur, we should stop and ask, “Did people institute these practices because they were beneficial, or were they forced on them?” Historians debate this very question, trying to determine whether civilization was a bottom-up or top-down development.

start superscript, 1, end superscript Most likely, it was a bit of both.^2

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Some people think civilization is an advanced stage in the progression of human cultural evolution. But, when historians or anthropologists use the term civilization, they mean a society has many different, interconnected parts. So, rather than thinking about different forms of social organization as completely separate models, it’s helpful to think in terms of a spectrum of complexity. On one end, we have hunter-forager societies—which have little complexity—and on the other end, we have civilizations—which are highly complex. In between lie a wide variety of social structures of varying types and levels of complexity.

Explanation:

the correct answer is d

new evidence is discovered.

oversights and errors are corrected,

new technology is applied to evidence.