The director of a fitness center wants to examine the effects of two exercise classes (spinning and aerobics) on body fat percentage. a six-week spinning class and a six-week aerobics class are offered at the same time and on the same days, so that a person can enroll in only one of them. a new class of each is about to begin, and each class has 25 people in it. ten people are randomly selected from each class. each person's body fat percentage is measured at the beginning and again at the end of the six-week class. using the change in body fat percentage as the response variable and conducting a test at the α = 0.01 level, the director determines that there is a significant difference between the treatment means. which of the following is a confounding variable in the study?
a) the director’s choice of spinning and aerobics classes as the types for use in the study
b) the random sample of 10 people from each class
c) the participants’ choice of which class to take
d) the use of body fat percentage as the measure of effectiveness of the treatment
e) the fact that both classes were conducted three times each week at the same time of day and for the same amount of time each day