From bottom of my heart, I would like to show my deepest gratitude
to many people helping and encouraging me enthusiastically in preparing and
completing the graduation paper.
First of all, I would like to express my heart felt thanks to Ms. Nguyen
Thi Thu Huyen, M.A., my supervisor, who has given me many valuable
guidance, helpful suggestions and advices for my research.
Also, I would like to express my gratitude to Ms. Nguyen Thi Yen
Thoa, M.A., for her help during my completion of the paper.
Next, I am also grateful to all teachers at Haiphong Private University
whose support and advices have improved my study.