Authors' contributions
Ming-zi An and Qing-he Zhou were major contributors to this study. Te-sheng Gao, Qing-he Zhou and Ming-zi An contributed to the study conception and design. Data were collected by Cheng-yun Xu, Yue-ru Hou and Zhen-ping Li. The data analysis was performed by Ming-zi An and Qing-he Zhou. The first draft of the manuscript was written by Ming-zi An. All authors read and approved the final version of the manuscript.