First of all, I would like to exPss my gratitude to all those who helped me during the writing of this thesis. I gratefully acknowledge the help of my supervisor, Ms**, who has offered me valuable suggestions in the academic studies. In the Pparation of this thesis， she has spent much time reading through each draft and provided me with inspiring advice. Without her patient instruction, insightful criticism and expert guidance, the completion of this thesis would not have been possible.
Second, I also owe a special debt of gratitude to all the professors in Foreign Languages Institute, from whose devoted teaching and enlightening lectures I have benefited a lot and academically Ppared for the thesis.
Last, I should finally like to exPss my gratitude to my beloved parents who have always been helping me out of difficulties and supporting without a word of complaint.