What was one of the biggest changes in western music in the twentieth century?

In contour drawing crossing lines is one way to indicate this

In 2012 it was reported that 64% of adult americans watch football regularly. you want to know if this percentage has increased. to do this you randomly sample 400 adults from across the united states. you ask all these adults, "do you regularly watch football? " 268 of the adults respond that they do regularly watch football. carry out at alpha =.05 a hypothesis to test your claim. make sure to check all necessary conditions and state your conclusion in the context of the situation

I’m a paragraph of no less than 125 words, explain the single most important factor involved in advancing american classical music in the twentieth century.