Please read the research report below: a A new study shows that people who eat chocolate frequently have lower body mass indexes than those who eat it less often. The researchers could not explain precisely why something loaded with sugar, fat and calories would have a beneficial effect on weight. But they suspect that antioxidants and other compounds in chocolate may deliver a metabolic boost that can offset its caloric downside. On average, participants in the study were middle-aged, exercised about three times a week and ate chocolate about twice a week. The people who ate chocolate the most frequently, despite exercising no differently from those who ate the least chocolate, tended to have lower Body Mass Indexes. There was a difference of roughly five to seven pounds between subjects who ate five servings of chocolate a week and those who ate none, Dr. Golomb said. a a. Describe the experimental design used here (e.g., two-group only-after; see Causality lecture). (.5 pt) b. Are there any threats to the validity of the authors' conclusions? If so, what are they? (1 pt) c. What would be a better experimental design to test the authors' hypothesis while controlling for some of the threats to validity? (1 pt)