A researcher would like to compare the effectiveness of two different methods for teaching third-grade mathematics. One group of students is assigned to Dr. Smith's class using method I and another group is assigned to Dr. Jones' class using method II. At the end of the school year, each child is given a standardized mathematics achievement test and the two groups of scores are compared to see which method produced a better outcome. What is the confounding variable in this study? a. the teaching methods b. nothing; there is no confounding variable in this study. c. the standardized test d. the teachers