First of all, the content and scope of this paper are appropriate, and the difficulty is moderate, which is more suitable for undergraduates.

Secondly, there are a lot of foreign literature research results, which laid the foundation for this study, and there are also many resources and materials available in libraries and networks. By consulting a large number of relevant materials, we can determine the correct viewpoints and methods, list, summarize and sort them out, draw scientific conclusions and complete the writing of graduation thesis.

Finally, through consulting the library and paper net's materials and documents, we can determine the correct viewpoints and methods.

And through classification, induction, sorting, draw scientific conclusions and complete the writing of graduation thesis.