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. 2009 Dec 24;4(12):e8467. doi: 10.1371/journal.pone.0008467

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Lu et al.

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.

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“n” represents the number of subjects in each area. Statistical data on the population of each area are from National Bureau of Statistics of China, updated in 2007 when samples were collected.