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Students in a large statistics class were randomly divided into two groups. The rst group took the midterm exam in the morning with a symphony by Mozart playing in the background while the second group took the exam in the afternoon with rock music playing. The scores of the two groups on the exam were compared. 15. This study is a(n) A) randomized comparative experiment. B) experiment, but without randomization. C) simple random sample. D) observational study, but not a simple random sample. 16. In this study, the response variable is A) the score on the midterm exam. B) the type of music that was playing during the exam. C) the placebo. D) the nal course grades of students. E) a lurking variable