What was the cause of the warring states period
A. China went to war against the Xiongnu and the Manchus to defend its territory around the great wall from invasions
B. A dynasty overtaxed the population during a major drought, so the states united together and fought a revolution
C. The feudal system ended after several lords declared their independence and formed states that battled for territory
D. Many people were outraged that the government refused to educate their children who were not in wealthy families