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Population Division, Department of Economic and Social Affairs, United Nations, New York,
US
Submitted: 1 December 2015 | Published: 1 December 2015
© 2015 by the Danan Gu. Licensee AccScience Publishing, Singapore. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution -Noncommercial 4.0 International License (CC BY-NC 4.0) ( https://creativecommons.org/licenses/by-nc/4.0/ )