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．
1． [A] when [B] why [C] how [D] what