This paper was completed under the careful guidance of xxx's tutor. My tutor's profound professional knowledge, rigorous academic attitude, work style of striving for perfection, noble ethics of tireless teaching, 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 did I set a lofty learning goal and master the basic research methods, but I also learned a lot about how to deal with people. From the topic selection to the completion of this paper, every step was completed under the careful guidance of the tutor, and a lot of painstaking efforts were devoted to the tutor. Here, I would like to express my high respect and heartfelt thanks to my tutor! In the process of writing a thesis, I encountered many problems, and all the problems were solved under the patient guidance of the teacher. So here, once again say to the teacher: thank you, teacher!