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.