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 Ms Hu, my supervisor, for her constant encouragement and guidance. She has walked me through all the stages of the writing of this thesis. Without her consistent and illuminating instruction, this thesis could not have reached its present form. Her useful advice and help made my paper much more mature than before. I benefit a lot from her valuable suggestions.
Second, I would like to express my heartfelt thanks to Ms Ge, who always help and encourage me in the study over the past three years. She gives me great confidence in learning English. I am also greatly indebted to the teachers at the Department of English: Ms Liu , Ms Zhao, Ms Huang and so on, who have instructed and helped me a lot in the school life.
Last my thanks would go to my beloved family for their loving considerations in me all through these years. I also owe my sincere gratitude to my friends and my fellow classmates for their kind assistance in the course of my preparing the thesis. I have benefited from discussions with them concerning both the subject and the content.