As Fascist leaders such as Mussolini and Hitler strengthened their rule in the 1930s, most citizens of the United States <span>wanted to remain free from the political affairs of Europe.</span>
Answer would be b. phanerozoic
This would be a perfect example of the system of barter as you are trading one book for the other. You can see from here what barter is, a trading system which involves direct exchange of goods without some kind of medium of exchange, for example money. Services can also be traded this way.