The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence. 

People on Pacific islands once exchanged shells for goods. The Chinese used cloth and knives. In Africa, elephant tusks or salt were used. Even today, some people in Africa are still paid in salt. Coins were first invented by the Chinese. They were round pieces of metal with a hole in the center.
In the past, many different things were used as（）