B because I just learned this in my World History class yesterday.
The answer is the Grand canal
Before WWI America owed Europe a lot of money, but after WWI the situation changed and the former allies owed America more than US$ 10 Billion for the cost of armaments and food supplies.
After the WWI a decade of tremendous prosperity started. This economic boom was a consequence of the mass production industries increasing gross domestic products, productivity and income, so business sales increase, driving up profits.
