文档介绍：Many people have offered me valuable help in my thesis writing, including my tutor, my classmates and my parents.
Firstly, I would like to give my sincere gratitude to Prof. Mu Jinjiang, my tutor who, with extraordinary patience and consistent encouragement, gave me great help by providing me with necessary materials, advice of great value and inspiration of new ideas. It is his suggestions that draw my attention to a number of deficiencies and make many things clearer. Without his strong support, this thesis could not been the present form. My heartfelt thanks also go to Prof. Xia hou, my teacher of thesis writing course , for his help in the making of this thesis as well as his enlightening lectures from which I have benefited a great deal.
Then, I pleased to acknowledge my classmates for their invaluable assistance throughout the preparation of the original manuscript. They graciously make ments and sound suggestions to the outline of this paper.
Besides, I am extremely grateful for my roommates for their cooperation, comments, and wise advice, although their advice is not always taken, where faults and infelicities remain.
Finally, in particular, I would like to express my gratitude to my parents for their support; they provided puters for me to search for information. It is of great help for me to finish this thesis essfully.
I would like to express my gratitude