Dietitian: Eating fish can lower one' s cholesterol level. In a study of cholesterol levels and diet, two groups were studied. The first group ate a balanced diet including two servings of fish per week. The second group ate a very similar diet, but ate no fish. The first group showed lower cholesterol levels, on average, than the second group. The two groups had displayed similar average cholesterol levels prior to the study. Which one of the following most accurately describes the role played in the dietitian's argument by the claim that the two groups had displayed similar average cholesterol levels prior to the study? ['It expresses the main conclusion of the argument.', 'It provides background information on the purpose of the study.', 'It introduces an alternative explanation of the phenomenon described in the main conclusion.', 'It rules out an alternative explanation of the data collected in the study.']