A two-way ANOVA compares two exercise programs (A and B) and two exercise frequencies (twice a week and four times a week). The response variable is weight lost after 8 weeks of exercise. An interaction is present in the data when
(a) the mean weight loss is not the same for Program A and Program B.
(b) the mean weight loss under Program A is different for twice-weekly and four-times-weekly subjects. (c) the difference between the mean weight losses for Programs A and B is not the same for twice-weekly and fourtimes-weekly subjects.