The passage suggests that one major difficulty in establishing patterns of migration is
A． the infrequency of life course events, which restricts the amount of data available to demographers
B． the overwhelming availability of proxies in migration patterns, which creates a degree of stasis in migration records
C． the lack of parishioner birth records, which limits the hard evidence upon which demographers base their observations
D． the homogeneity of single parishes, which makes it difficult to distinguish the motivations of migrants
E． the repetitive nature of migration, which results in a surfeit of unusable data that overwhelms demographers