1, research background and purpose: This part briefly introduces the background of the current research field, and expounds the present situation and shortcomings of previous studies, as well as the purpose and significance of this study.

2. Summary of previous studies: This part needs a comprehensive summary of previous studies, including the main contents and achievements of previous studies and the shortcomings of previous studies. In the process of review, it is necessary to objectively evaluate the viewpoints, methods and conclusions of previous studies, so as to provide reference for subsequent studies.