Historically, monetary systems have developed only in population centers with marketplaces. Through the fourth century B. C. , Mesopotamian cities engaged in trade, but had never had marketplaces. By that period, however, Greek cities all had marketplaces, or agorae. The Greek cities' agorae were centrally located and goods were traded there either for money or for commodities.
If all of the statements in the passage are true, then it must be true that Greek cities were the only population centers with monetary systems in the fourth century B. C.