Crosby’s recent study of American historical demography is blithely based on the reconstitution of the records of single parishes, a method that often excludes migrants. Moreover, it is troublesome for historians to obtain information on the birthdates of people who relocated to the parish, and equally difficult to follow those who had migrated to new places of residence. Thus, the exclusion of migrants also followed from the way spatial units were once conceived by the parishioners themselves, a stable and unchanging pre-modern countryside of interchangeable towns unlike "modern" flows to cities. As a result, migration was improperly assumed to be irrelevant because the small units in the countryside were interchangeable and migrants into a parish could thus stand as a proxy for those who had left. In any case, it was thought that migration in the countryside was repetitive and occurred only in response to life course events, such as finding a spouse, and thus, like the parishioners themselves, Crosby complacently equates the demographics of migrants to those of more sedimentary populations. According to the passage, Crosby has made which of the following assumptions concerning historical means of demography？（）

A. Migration is in most cases dictated by life course events as opposed to the economic factors that contribute to the development of cities.
B. Population growth tends not to affect the availability of proxies for replacing emigrants from population centers.
C. Sedimentary populations are more historically significant than nomadic or migratory ones.
D. It is permissible to rely upon a single source of information in studying population movement patterns.
E. Migration can be disregarded as a demographic pattern in historical contexts prior to the development of large cities.