填空题

The history of money is very interesting. (11) were among the earliest objects used as money. They were originally used as ornaments, but gradually people in the Pacific began to use them (12) food and other items. Many other objects were also used as money. For example, rice was used in the Philippines , whereas (13) was used in Africa and is still used as money today in certain parts of that continent. Coins came in many shapes and (14) . They were (15) copper in Russia. However, their weight was the main factor that led to the invention of paper money.