Why can the period from 1877 to wwi be called the beginning of the american century?

The American Century is a characterization of the period since the middle of the 20th century as being largely dominated by the United States in political, economic, and cultural terms. It is comparable to the description of the period 1815–1914 as Britain's Imperial CenturyExplanation:

answer; i believe the correct answer is ( quakers);

b. the 200th birthday of the united states