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Polk has textbook. Polk’s textbook is the most official reading book assigned by the school, which is based on the content of the school curriculum. Usually in the lower grades of primary and secondary schools, students are prepared for tests and examinations as long as they are familiar with the textbook content. Textbooks are also important teaching tools for teachers. More than 90% of teachers in the United States use textbooks for teaching in 90% of their teaching time. In the growth of knowledge, the textbook's credibility is to have certain authority. At least the test answers are textbook based. Some scholars believe that the school's absolute right to choose textbook market makes textbook publishing company's operating costs increase day by day.