(第12段)Of course(当然), the investigators(调查者) say, social networks (人际关系网)(与选项A中的social contact/社会接触呼应)are not the only(唯一的，仅仅) factors (问题句中的核心词)that affect body weight(体重). There is a strong(强烈的，强大的， 浓的) genetic (遗传的)component(成分) at work(在工作， 起作用), too(也， 太).
答案相关段(第十二段)提到了遗传因素和社会接触(人际关系网的形式)对体重的作用和影响。同时间接地隐含了环境因素对人的体重起很重要的作用。至于C(生活方式)，文中并没有提到。
5. In what way is obesity(肥胖) contagious(传染性的) and epidemic(会流行性的)?
A. social networks(人际关系网) let the obesity(肥胖) spread(伸展， 展开， 蔓延) rapidly(迅速地)
B. individuals(个人) have genetically(遗传方面地) determined(决定了的) ranges(范围) of weights(重量，体重).
C. Obesity(肥胖) can easily(容易地) spread from one to another (从一个到另一个)within any physical contact(身体接触)
D. Obesity can spread rapidly(迅速地) and extensively(广泛地，广阔地) by infection(传染，传染病) and affecting(影响) many individuals(个人) in an area(地区， 范围) or a population(人口) at the same time(同时地).
5. A. 利用问题句中的修饰词(contagious, epidemic ) 作为答案线索， 在文章中查找答案相关句：
Obesity can spread from person to person, much like a virus, researchers are reporting today. When one person gains weight, close friends tend to gain weight, too.
Their study, published in The New England Journal of Medicine, involved a detailed analysis of a large social network of 12,067 people who had been closely followed for 32 years, from 1971 to 2003.
The investigators knew who was friends with whom as well as who was a spouse or sibling or neighbor, and they knew how much each person weighed at various times over three decades. That let them reconstruct what happened over the years as individuals became obese. Did their friends also become obese? Did family members? Or neighbors?
The answer, the researchers report, was that people were most likely to become obese when a friend became obese. That increased a persons chances or becoming obese by 57 percent. There was no effect when a neighbor gained or lost weight，however, and family members had less influence than friends.
It did not even matter if the friend was hundreds of miles away, the influence remained. And the greatest influence of all was between close mutual friends. There, if one became obese, the other had a 171 percent increased chance of becoming obese, too.
The same effect seemed to occur for weight loss, the investigators say. But since most people were gaining, not losing, over the 32 years, the result was, on average, that people grew fatter.
Dr. Nicholas A. Christakis, a physician and professor of medical sociology at Harvard Medical School and a principal investigator in the new study, said one explanation was that friends affected each others’ perception of fatness. When a close friend becomes obese, obesity may not look so bad.
“You change your idea of what is an acceptable body type by looking at the people around you,” Dr. Christakis said.
The investigators say their findings can help explain why Americans have become fatter in recent years – each person who became obese was likely to drag along some friends!
Their analysis was unique, Dr. Christakis said, because it moved beyond a simple analysis of one person and his or her social contacts and instead examined an entire social network at once, looking at how a person's friend's friends, or influence on a person's weight or a spouse's sibling's friends, could have an influence on a person’s weight.
(倒数第4段)The effects(影响), he said, “highlight(强调， 突出) the importance(重要性) of contagion(传染， 传染病), that spreads(蔓延) through the network.”
Of course, the investigators say, social networks are not the only factors that affect body weight. There is a strong genetic component at work, too.
Science has shown that individuals have genetically determined ranges of weights, spanning perhaps 30 or so pounds for each person. But that leaves a large role for the environment in determining whether a person’s weight is near the top of his or her range or near the bottom. As people have gotten fatter, it appears that many are edging toward the top of their ranges. The question has been why.
(最后一段) If the new research(研究) is correct(正确的), it may say(说) that something in the environment(环境) seeded (种子， 播种)what some call(称呼) an obesity (肥胖)epidemic(流行病 )， making a few people gain weight(体重增加). Then(然后) social networks (人际交际网)let the obesity spread rapidly.
答案相关句(文章最后一段)直接说明了“人际关系网”是造成肥胖流行病蔓延的主要原因，因此A是答案。
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