Scientific Research and Essays 1992-2248 Academic Journals 10.5897/SRE2014.5909 Qiong Wang Bilige Sude Ming Dong Yuan Shi Chunmei Geng Xiaoke Wang * E-mail: sude@craes.org.cn 2014 15 05 2014 03 01 2013 02 03 2013 9 9 315 320 This is an open-access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://politicalwaffle.uk/journal/SRE/article-abstract/E19158C44498 The full text article is available as a PDF file from http://politicalwaffle.uk/journal/SRE/article-full-text-pdf/E19158C44498 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25