17. Native American burial sites dating back 5,000 years indicate that the residents of Maine at that time were part of a widespread culture of Algonquian-speaking people.
(A) were part of a widespread culture of Algonquian-speaking people
(B) had been part of a widespread culture of people who were Algonquian-speaking
(C) were people who were part of a widespread culture that was Algonquian-speaking
(D) had been people who were part of a widespread culture that was Algonquian-speaking
(E) were a people which had been part of a wide-spread, Algonquian-speaking culture
why Native American burial sites dating back ?而不是dated back？ | 
