Which choice most logically completes the text?

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Northeastern Kansas's Jefferson County is among the most rural counties in the United States: the US Census Bureau classified it as 98.8% rural in 2010. Researchers studying populations of counties like Jefferson often struggle to recruit and retain participants. Melissa Valerio and colleagues tested whether a method called snowball sampling could improve recruitment and retention. Working in two rural counties, the researchers identified a small number of people who had the characteristics desired for a proposed study and asked them to recruit additional participants from their social networks. Valerio and colleagues found that participants recruited via snowball sampling showed a much higher retention rate than did people recruited by strangers, suggesting that

being recruited to participate in a study by someone with whom one is socially connected may impart a feeling of obligation to persist with participation in the study.

people with relatively small social networks are inherently less likely to be recruited to participate in a study via snowball sampling than are people with relatively large social networks.

snowball sampling is more likely to improve retention rates among rural participants than among nonrural participants.

social networks can become large enough that two people can share a network but nevertheless regard each other as strangers.