After several drafts, I finally finished the writing of my master's thesis. In the final stage of writing my thesis, I often think of my teachers, classmates, relatives and friends who have guided me, helped me, cared for me and taken care of me over the past few years. In the past three years, I have made their efforts for my study and thesis research. Only by expressing my deep gratitude to them can I express my gratitude.

In particular, I would like to thank my tutor Professor Xie Zhihua for his patient guidance and cordial care during the course learning stage and the process of topic selection, writing and revision of the thesis. His profound knowledge, rigorous academic attitude, scientific working methods, strict with self-discipline and lenient with others have benefited me all my life!

In a blink of an eye, my study and life in Beijing Technology and Business University passed like this, and three years left me with too many wonderful memories. Excellent teachers and enthusiastic classmates, beautiful campus environment and positive learning atmosphere make me full of infinite gratitude to the school!

Finally, I want to thank my family in particular. In order to successfully complete my graduate studies, their constant encouragement and support always make me full of confidence. Their expectation and understanding are the driving force for me to keep moving forward.

I want to express my gratitude to many authors of the materials cited in this article. There are many people to thank, and they are always worried about missing something. I thank all those who help and care about me.

Time flies, I will always miss this life journey full of hope, melancholy, enrichment and forge ahead!