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：名词辨析题。空格处的句子在进一步说明这项研究的结论。前文提到朋友之间有1%的基因相似性、朋友间相似的是嗅觉基因以及相似的基因进化速度快，可知人们在选择朋友时倾向挑选和自己基因相似的人。此处描述的是一种倾向性或趋势，再根据空格处语义，这些发现并不仅仅是在解释人们交友时的倾向。故本空应填入tendency。

错项排除：由原文主题可知，人们在选择和自己亲属相似的人做朋友时是一种不由自主的行为，而并非主观或刻意选择的。其余三个选项意思都含有主观的意思，故排除。