In his speech about the rising cost of textbooks, Eli said, "For example, let’s imagine that there is a college student named Joe. Joe is taking public speaking, composition, algebra, and biology this semester. If Joe had walked into the bookstore to buy his textbooks in 1985, he probably would have paid around $400 for his books, but today, Joe’s books would probably cost around $800." What kind of support material is Eli using?