I want to express my appreciation to the people who supported my efforts in writing this paper during the past few months.
I am greatly indebted to express my gratitude to those who have helped me and given valuable suggestion to me. Hearty thanks should also be given to my dear tutor Ms. Bai who has provided precious instruction and valuable advice. Her comments and corrections are the chief factors in my success. Without her, it would be impossible for me to complete it.
Special thanks should be extended to my beloved family and friends who constantly encourage and enlighten me in finishing my paper.