Epidemiologists now consider obesity to be a global pandemic, occurring largely as a result of the spread of technology and culture worldwide.  Obesity has been linked to many serious health conditions, and as the obesity rate rises, the overall poorer health of the population impairs the functioning and stability of society.  Although experts disagree about whether obesity itself should be viewed as a disease to be treated with medical interventions, there is widespread consensus that obesity prevention is critical.  Research has shown that across one's life span, multiple biological, psychological, and social influences contribute to obesity; understanding the influence of these factors is essential to the development of effective obesity prevention programs.Study 1 was conducted using nearly 6,000 residents from a mid-sized city to investigate how obesity rates change over time in informal social groups.  During recruitment wave A, researchers asked an initial group of individuals to complete a two-minute health questionnaire; those who completed the questionnaire were then asked to participate in the research study.  During recruitment wave B, additional participants were recruited using a snowball sampling technique:  participants who enrolled during wave A provided contact information of friends and family members who were then asked to participate.  Researchers finalized the sample such that each participant had at least one family member, one close friend, and one acquaintance also in the study.  Every year for 20 years, researchers measured each participant's height and weight to calculate body mass index (BMI) and conducted an extensive interview with each participant to assess the type, strength, and geographic distance of their social ties with other participants.  Researchers found that the type and strength of social ties appeared to be more strongly linked to BMI than geographic closeness between social ties.Study 1 was criticized for not addressing larger contextual variables related to obesity, such as the availability of unhealthy food within a community.  Therefore, Study 2 randomly selected a subset of 500 cases from the Study 1 database and, using participants' addresses, researchers mapped and counted the number of retail food chains (ie, "fast food" restaurants selling low-quality, calorie-dense food) located within a 1-mile radius of participants' homes and compared this to BMI at the end of the 20-year study.  Results showed a significant relationship between higher BMI and more neighborhood retail food chains.J. A. Levine ©2011 American Diabetes Association; Christakis, N. A. and Fowler, J. H. ©2007 Massachusetts Medical Society Question 30Regarding Study 1, which of the following statements is most accurate?  Study 1:A.determined the causal mechanisms of weight gain.B.used a mixed methods design.C.avoided sampling bias.D.controlled for all confounding variables.