A researcher is interested in determining if teaching delivery affects material retention as measured by an exam. She takes a total of 90 volunteer students and randomly splits them into three groups. The students in Group 1 are taught a short French lesson through five short in-person class meetings. The students in Group 2 are taught the same French lesson through five short online but "live" (Zoom) class meetings. The students in Group 3 are taught the same French lesson through five short pre-recorded videos. All students are assessed on what they learned from the lesson with a quiz on the material. a) Justify why this is a controlled experiment and not an observational study. b) Is there a control group in this study? c) What is the explanatory variable in this study? The response variable