Textbook Interpretation and Development is also a synchronous supplementary book. In that case, there should be, but now the new college entrance examination reform and so on, some textbooks say that they need to be changed, so the supplementary books are under pressure, especially the synchronous supplementary books, because they have to be matched with the textbooks and courses, so only the new textbooks will come out.

Another situation is that because 4-4 is an elective course with less content, it will not be published separately or in an educational edition. I used a complete interpretation of the textbook, that's all. That is, 4-4 and 4-5 are combined into one book, and it is not a special edition of PEP, but a general edition based on PEP, which integrates the contents of other editions. Most elective courses are like this.