单选题 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