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.
The statement "The Mesopotamian cities of the fourth century B.C. did not have monetary systems" must also be true if all of the statements in the passage are true.