According to the passage, which one of the following is true about the focus of historical studies on ethnic groups in the United States? A. Current studies are similar to earlier studies in claiming that a sense of peoplehood helps preserve ethnic community. B. Current studies have clearly identified factors that sustain ethnic community in generations that have been exposed to the pluralism of American life. C. Current studies examine the cultural practices that make up the American national character. D. Earlier studies focused on how ethnic identities became transformed in the United States. E. Earlier studies focused on the factors that led people to immigrate to the United States.