Background and significance of topic selection: explain the historical background, research status and development trend at home and abroad. On the one hand, writing these contents in the opening report can highlight the status and value of this study [3], that is, the significance of the topic, including the explanation of the theoretical and practical significance of the topic; On the other hand, it can also explain whether the author of the opening report has grasped this research well. Specifically, go to the paper net of Sanqi (www.lw777.net). There are many free sample papers on their website for reference ~ Great!