【单选题】

In ancient times wealth was measured and exchanged in things that could be touched: food, tools, and precious metals and stones. Then the barter system was replaced by coins, which still had real value since they were pieces of rare metal.Coins were followed by fiat money, paper notes that have value only because everyone agrees to accept them.

Today electronic monetary systems are gradually being introduced that will transform money into even less tangible forms, reducing it to a series of "bits and bytes", or units of computerized information, going between machines at the speed of light.Already, electronic fund transfer allows money to be instantly sent and received by different banks, companies, and countries through computers and telecommunications devices.
Aeeording to the passage,coins once had real value as currency beeause they ______.A．represented a great improvement over barter
B．permitted easy transportation of wealth
C．were made of precious metals
D．could become collector’s items