The first thing I want to thank for the completion of the thesis is my tutor. From the topic selection to the finalization, it is inseparable from the obedience and teaching of xxx teacher. At the beginning of thesis writing, I have been wandering outside the academic door as a layman. Teacher xx taught me by hand, which made me feel the academic charm and the teacher's concern for me, and strengthened my future learning direction. Here, I would like to express my most sincere thanks and respect to Teacher xxx. Thank my parents and girlfriend Xiao Xiong for their infinite care in life, especially Xiao Xiong. When I fell in love with her, I didn't cherish her love. I regretted losing her. Maybe I have no chance to make up for it in my life. When I am about to graduate, I deeply regret to say to you: "Your love has made me grow up and let me know about love, but what I left you is scarred, and you have already left." If there is an opportunity, I will say: "Finally, thank all the teachers, classmates and friends who care about me and have helped me!"!