Aschool district decides to compare a new math textbook to the textbook that has been in use for the past few years. each of the fourth graders in one school is assigned to be in one of two classes, one in which a teacher continues using the old book and another in which a new teacher uses the new book. the school administrators compare the average scores of the two classes on a mathematics standardized test at the end of the year to determine which book is better. which of the following is a potential confound in this study?