Why do you think americans supported participation in the un after wwii when they had opposed participation in the league of nations after wwi?

After WW1, most Americans were in denial of their place in the world. They went on a period of "isolation" from Europe. After WW2, they realized what President Wilson said was correct: that another World War would happen, even worst than the first, if we do not take action against it. America would have to participate in settling world problems if another Great War was to be prevented. 

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The accomplishment by the Song dynasty of China is that the song rulers expanded the use of coinage and paper currency.

Explanation

The period of Song dynasty was the golden period of China with several advances in technology, art and administration. Earliest  form of paper money in the world developed during the period of Song rulers and it was called Jao Zi. It was one of the popular paper currency of that time.

The high development of commerce is indicated by this factor. The Song period was also famous for the technological advances. The world famous inventions, compass,gun powder and movable printer was in China during the Song period.