In the Earliest Days of Cities, What Was the Key

In the earliest days of cities, what was the key distinction between city dwellers and rural dwellers?

A) City dwellers had lower birth and death rates.
B) All rural people were involved in agricultural production, but not all city people were.
C) Rural dwellers had better protection from the elements than did city dwellers.
D) City dwellers had better access to fresh water than did rural dwellers.