Are you a social butterfly, or do you prefer being at the edge of a group of friends? Either way, your genes and evolution may play a major 1 , US researchers reported on Monday.
While it may come as no surprise that genes may help explain  2  some people have many friends and others have  3  , the researchers said, their findings go just a little farther than that.
"Some of the things we find are 4   uncommon," said Nicholas Christakis of Harvard University in Massachusetts, who helped  5 the study.
"We find that how interconnected your friends are  6   on your genes. Some people have four friends who know each other and some people have four friends who don't 7 each other.  8     Dick and Harry know each other depends on Tom's  9    ," Christakis said in a telephone interview.
Christakis and colleague James Fowler of the University of California San Diego are  10    known for their studies that show obesity, smoking and happiness spread in networks.
For this study, they and Christopher Dawes of UCSD used national data that  11  more than 1,000 identical(同卵的) and fraternal（异卵的） twins’ genes. Because  12  share an environment, these studies are good for showing the impact that genes have 13  various things, because identical twins 14  all their genes while fraternal twins share just half.
"We found there appears to be a genetic tendency to introduce your friends 15  each other," Christakis said.
There could be good, evolutionary reasons 16  this. People in the middle of a social network could be secret to useful gossip,  17  the location of food or good investment choices.
But they would also be at risk of catching effects from all sides -- in which case the advantage would 18  more cautious social behavior, they wrote in the Proceedings of the National Academy of Sciences.
"It may be that natural selection is  19 not just things like whether or not we can resist the common cold, but also who it is that we are going to come into  20   with," Fowler said in a statement.
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(    ) 2. A. where              B. why          C. when                D. how
(   ) 3. A. a few                      B. several      C. few                  D. some
(    ) 4. A. generally          B. mainly      C. mostly              D. frankly
(    ) 5. A. conduct            B. introduce   C. conflict            D. instruct
(    ) 6. A. depends            B. bases         C. take                 D. put
(    ) 7. A. see                   B. inspect      C. learn                D. know
(    ) 8. A. When               B. Where       C. What                D. Whether
(    ) 9. A. genes                      B. brains        C. appearances      D. figures
(   ) 10. A. much             B. best           C. very                 D. least
(   ) 11. A. recorded         B. illustrated C. described          D. compared
(   ) 12. A. brothers          B. sisters        C. twins                D. cousins
(   ) 13. A. with                      B. on             C. for                   D. to
(   ) 14. A. share              B. have          C. own                 D. show
(   ) 15. A. into                B. to             C. in                    D. from
(    ) 16. A. for                 B. with          C. to                    D. at
(    ) 17. A. in addition to B. due to       C. as for                      D. such as
(   ) 18. A. lie on             B. bring in     C. lie in                D. send in
(    ) 19. A. acting on               B. putting on C. relying on         D. sending on
(    ) 20. A. contract          B. face          C. join                  D. contact