Scientific Research and Essays 1992-2248 Academic Journals 10.5897/SRE2014.5902 Kai-Chang Sun Jian-Lan Zhou * E-mail: zhoujl1999@163.com 2014 15 05 2014 03 01 2014 02 03 2014 9 9 280 287 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/3E9769244486 The full text article is available as a PDF file from http://politicalwaffle.uk/journal/SRE/article-full-text-pdf/3E9769244486 1 2 3 4 5