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Question 58·200 Super-Hard SAT Reading Questions·Expression of Ideas

Archaeologists once attributed the disappearance of a coastal city to a single catastrophic tsunami. New sediment cores, however, show multiple thin layers of marine sand separated by decades of urban debris, indicating repeated flooding events rather than one calamity. ______ the evacuation of the city appears to have been gradual, as wealthier districts were abandoned first while poorer neighborhoods continued to be occupied for years.

Which choice completes the text with the most logical transition?