Though not biologically related,friends are as“related”as fourth cousins,sharing about 1%of genes.That is_（1）_a study,published from the University of California and Yale University in the Proceedings of the National Academy of Sciences,has__（2）_.The study is a genome-wide analysis conducted_（3）__1,932 unique subjects which__（4）__pairs of unrelated friends and unrelated strangers.The same people were used in both_（5）_.While 1%may seem_（6）_,it is not so to a geneticist.As James Fowler,professor of medical genetics at UC San Diego,says,“Most people do not even_（7）_their fourth cousins but somehow manage to select as friends the people who_（8）_our kin.”The study_（9）_found that the genes for smell were something shared in friends but not genes for immunity.Why this similarity exists in smell genes is difficult to explain,for now,_（10）_,as the team suggests,it draws us to similar environments but there is more_（11）_it.There could be many mechanisms working together that_（12）_us in choosing genetically similar friends_（13）_”functional Kinship”of being friends with_（14）_!One of the remarkable findings of the study was the similar genes seem to be evolution_（15）_than other genes Studying this could help_（16）_why human evolution picked pace in the last 30,000 years,with social environment being a major_（17）_factor.The findings do not simply explain people’s_（18）_to befriend those of similar_（19）_backgrounds,say the researchers.Though all the subjects were drawn from a population of European extraction,care was taken to_（20）_that all subjects,friends and strangers,were taken from the same population.
17选？
上下文语义衔接+形容词辨析此空需要填入一个形容词，表达社会环境对人类进步所起到的作用。Help，pick pace这两个表达都是积极的含义，因此此处需要填入一个带有褒义的感情色彩的词，因此排除A选项“不可预测的”和D选项“破坏性的”;B而C选项“可控的”,与语意无关；因此正确答案为B选项contributory“促进的”。
