【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.
The author mentions food, tools and precious metals and stones together because they are all ______.A．useful items
B．articles of value
C．diffieuh things to obtain
D．material objects