The stimulus tells us that one group of patients showed a reduction in cholesterol and triglycerides when taking garlic tablets, while the other group that took an inert tablet showed little change. This evidence is used to conclude that eating garlic reduces the levels of cholesterol and triglycerides in the blood. The issue with this argument is that we have limited information about the study participants and whether there are any confounding factors that might have influenced their levels of cholesterol and triglycerides. Since diet has a significant impact on these levels, we would want more information on the diets of each group during the study period as (B) correctly points out. If participants in the garlic tablet group generally had healthier diets with less fat/ cholesterol, it would be equally likely that the resulting reduction in their cholesterol levels was due to their diet rather than taking garlic tablets, undermining the argument.
(C) could provide additional information on whether taking garlic tablets on a short term basis is effective but it is not helpful in evaluating the results of this particular study where participants took tablets for a period of four months, thus it is out of the scope of the given argument.
