Which one of the following best summarizes the main point of the author of the passage? A. Fugita and O'Brien's study provides a comparison of the degree of involvement in ethnic community of different groups in the United States. B. Fugita and O'Brien's study describes the assimilation of three generations of Japanese Americans to United States culture. C. Fugita and O'Brien's study illustrates both a recent trend in historical studies of ethnic groups and a problem typical of that trend. D. Historical studies of ethnic preservation among Japanese Americans have done much to define the interpretive frameworks for studies of other ethnic groups. E. Historical studies are more concerned with the recent development of ethnic communities in the United States than with the process of adaptation to United States culture.