AcknowledgmentsFirst of all, I would like to thank my thesis instructor, teacher Lv. My teacher is a friendly and approachable person, and she always takes her work seriously and responsibly. Although I have a lot of problems in the process of writing the paper, my teacher still does not abandon me and she is not to give up on me. My teacher is always very careful to correct my mistakes, gives me timely feedback and the right guidance. I'm afraid I can't finish this paper well without my teacher's teaching. It is beyond words for me to express my gratitude to my teacher.Secondly, I would like to thank all the teachers who have taught me in the past four years. In these four years, they give me not only a wealth of knowledge, but also the correct direction of life. In philosophy there is a saying that the accumulation of quantity causes qualitative change, it is their daily teaching that makes me form a perfect personality. I am very grateful to them for teaching me at a crucial point in my life.Thirdly, I really appreciate my family and friends. The last year of college is another important turning point in my life. I decided to take the postgraduate examination, so that I could go on further study in my study. In this year, my family and friends gave me a lot of encouragement and care. It is what they do for me that make me more determined to pursue my goals in the days of preparation. At the same time, I thank myself for holding on to the end.Finally, I'd like to thank my adviser again. Because of various reasons, I do not do well in the process of thesis writing, and may even make my teacher a little disappointed. In spite of this, my teacher is still very ...