I would like to take this opportunity to appreciate all those who helped me during writing this thesis. My deepest gratitude goes first and foremost to Mr. Yang, my supervisor, for his constant encouragement and guidance. He has walked me through all the stages of the writing of this thesis. Without his consistent and illuminating instruction, this thesis could not have reached its present form. His useful advice and help made my paper much more mature than before. I benefit a lot from her valuable suggestions.