SAT Question 9e34720b: Transitions answer and explanation

English - Transitions - Cause Effect Transition
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Although those who migrated to California in 1849 dreamed of finding gold nuggets in streambeds, the state’s richest deposits were buried deeply in rock, beyond the reach of individual prospectors. ______ by 1852, many had given up their fortune-hunting dreams and gone to work for one of the large companies capable of managing California’s complex mining operations.
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Difficulty:Medium
Which choice completes the text with the most logical transition?