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 which one of the following must also be true? ['In the fourth century B. C. , the Greeks and the Mesopotamians traded with each other.', 'The Mesopotamian cities of the fourth century B. C. did not have monetary systems.', 'The development of monetary systems has historically led to the development of marketplaces.', 'In the fourth century B. C. , Greek cities were the only population centers with monetary systems.']