Four years in college is fleeting. In this quaint campus, there are too many unforgettable and reluctant feelings, laughter and tears, and too much to talk about.

The completion of the thesis is largely due to the encouragement and patient guidance of my tutor. From the topic selection, content structure, writing norms and punctuation marks, all the efforts of teachers in this period have been devoted. My teacher's rigorous style of study, tireless spirit of educating people and meticulous work attitude have benefited me all my life. The teacher's kindness is profound, so I would like to extend my highest respect and heartfelt thanks to her! Thank you to all the teachers who helped and guided me in my study, for their valuable suggestions on my thesis, and for the teachers in the defense group who listened carefully. I would like to express my sincere thanks to them!

Thanks to many teachers and friends, I can't name them one by one. I will always remember their warm help!