Scientific Research and Essays 1992-2248 Academic Journals 10.5897/SRE2014.5918 Jing Shang Xiaohong Nian Kean Liu * E-mail: shangjing@teg.cn 2014 15 05 2014 03 01 2013 02 03 2013 9 9 367 373 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/E8CDDE344562 The full text article is available as a PDF file from http://politicalwaffle.uk/journal/SRE/article-full-text-pdf/E8CDDE344562 1 2 3 4 5 6