那么本文能写到这里也就意味着开始收尾了。在这里心情有点复杂。从刚开始从指导老师那拿到选题，到毕业论文的完成经历了四个多月时间。真正开始着手查阅相关文献并跟指导老师...

Acknowledgements First of all, I want to express my special thanks to He Shenying and Wang Wenying, my supervisors, for their helpful suggestions in terms of the content and style of this paper. Without their patient assistance and friendly...