Acknowledgements
First and foremost, I would like to express my heartfelt gratitude to my supervisor, Mr.Zhong, both for his tellectual guidance and for his warm and constant encouragement during the process of writing this thesis. I am also very grateful to my classmates and students, who have given me a lot of help and courage during writing this thesis. Last but not the least, big thanks go to my family who has shared with me my worries, frustrations, and hopefully my ultimate happiness in eventually finishing this thesis.