This thesis was completed under the careful guidance of my tutor Professor * * and researcher * *. My tutor's profound professional knowledge, rigorous academic attitude, work style of striving for perfection, noble ethics of tireless teaching and educating people, noble demeanor of being strict with oneself and lenient with others, unpretentious and approachable personality charm have a far-reaching influence on me. Not only has he set up lofty academic goals, mastered basic research methods, but also learned a lot about treating people with things and dealing with others. From the topic selection to the completion of this paper, every step was completed under the guidance of the tutor, who devoted a lot of effort. Here, I would like to express my high respect and heartfelt thanks to my tutor!

The successful completion of this paper can not be separated from the care and help of teachers, classmates and friends. Thanks to Professor * *, Miss Li * * and Teacher * * for their guidance and help. Thanks to the guidance and help of the reference room teacher. Thank * * University for its concern, support and help;

During my study, I deeply appreciate the care and help from my brothers and sisters. Without their help and support, I can't finish my dissertation, and the friendship between my classmates will last forever.