The passage tells us that economists typically assume that people migrate if their expected earnings exceed existing earnings enough to outweigh the difficulties and one-time costs of migration (lines 25-40), thus suggesting that information about expected financial gains alone is insufficient to conclude whether an individual would migrate without considering the costs and difficulties associate with the migration and weighing them against each other. (A) correctly concludes that expected financial gains alone are not a reliable indicator.
(B) is incorrect because any migration during the 19th century is out of scope of the passage, the passage is primarily concerned with the Great Migration of the twentieth century, starting in 1915, and its drivers.
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