Crosby’s recent study of American historical demography is blithely basedon the reconstitution of the records of single parishes, a method that oftenexcludes migrants. Moreover, it is troublesome for historians to obtainLine information on the birthdates of people who relocated to the parish, and equally(5) difficult to follow those who had migrated to new places of residence. Thus, theexclusion of migrants also followed from the way spatial units were onceconceived by the parishioners themselves, a stable and unchanging pre-moderncountryside of interchangeable towns unlike "modern" flows to cities.As a result, migration was improperly assumed to be irrelevant because the(10) small units in the countryside were interchangeable and migrants into a parishcould thus stand as a proxy for those who had left. In any case, it was thoughtthat migration in the countryside was repetitive and occurred only in response tolife course events, such as finding a spouse, and thus, like the parishionersthemselves, Crosby complacently equates the demographics of migrants to those(15) of more sedimentary populations. In the passage, the author is primarily concerned with（）

A. summarizing the findings of a study
B. placing new research within its historical context
C. evaluating the methodology of a historian
D. comparing various demographical techniques
E. establishing categories