Tóm tắt nội dung: Acknowledgements The completion of this graduation research owns the help of people who have enthusiastically contributed in different ways. First and foremost, I would like to express my gratitude to my supervisor, Mr. Nguyen Duc Thang, M.A. for his enthusiastic and usual guidance and encouragement. His valuable suggestions, and careful and critical comments have been vital to the completion of this research. I would like to give my...