I feel grateful to all the teachers in the College of Foreign Languages of China University of Petroleum who once offered me valuable courses and advice during my study. My sincere thanks are also given to Pro. Dai Ruiliang and Pro. Sun Chengrong. from whose lectures I benefited greatly.

　　Last but not least, I owe much to my friends and classmates for their valuable suggestions and critiques which are of help and importance in making the thesis a reality.