1.A “calendar stick” carved centuries ago by the Winnebago tribe may provide the first evidence that the North American Indians have developed advanced full-year calendars basing them on systematic astronomical observation. (A) that the North American Indians have developed advanced full-year calendars basing them (E) that the North American Indians developed advanced full-year calendars based
答案是E,没问题,我想问,A选项中的them语法上优先指代什么??是North American Indians还是calendars???