AcknowledgmentsThis paper would not have been completed without the invaluable help and support of many people
I would like to take this opportunity to show my heartfelt thanks to all of them
My deepest gratitude goes first and foremost to my supervisor, Dr
Yang Shizhou
He gave me inspiring encouragement and illuminating instructions all through my writing
It is he who has spent much time reading through each draft and provided me with inspiring advice
Without his consistent and illuminating instruction, this paper could not have reached its present form
Secondly, I would like to express my thanks to Professor Li Qiang and every teacher who has helped me with useful advice and comments
My thanks also go to my friends and my fellow classmates, who always encouraged me when I felt upse