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 that the similar genes seem to be evolving (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.

答案精析：形容词辨析题。空格所在句的逗号后面with引导了一个伴随状语，用于对该句的前半部分进行补充说明。空格前的help和picked pace都是表达一种积极的意思，由此可知，本空也应填入一个意义相近的词来表示对前半句的补充。故本空应填入带有褒义感情色彩的词contributory（促进的）。

错项排除：A项unpredictable、C项controllable和D项disruptive都和原文意思无关，故排除。