There are two types of twins: identical (同性的) and non-identical twins. Identical twins are formed 1 a single egg in the mother's body which 2 to form two separate babies. Identical twins look the same, and are often 3 by their parents in clothes of the same colors. It is often 4 to tell identical twins from each other, even when they are standing 5 . Non-identical twins come into being when the mother 6 two separate eggs at the same time, both of 7 grow to form babies. 8 case the twins look like ordinary brothers and sisters and are easy to tell one from 9 .

　　In the 1970s and 1980s a scientist did some 10 into twins in the USA．He invited many 11 of identical twins to the university and asked them to 12 a week of tests. He was particularly 13 adopted (被收养的) twins who had been separated at birth. He 14 give the twins different kinds of tests to 15 their speed of thinking, their speech, their memory (记忆), 16 they saw and heard different things, and so on. 17 he found separate twins preferred clothes of the same color. They even used shaving (修面的) soaps of 18 kind.

　　There is a third type of twins. 19 , it is a very unusual one. Twins which are joined together at birth are known in western countries 20 Siamese twins.

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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.

(    ) 1. A. role                 B. rule           C. roll                  D. pole

(    ) 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

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.

(    ) 1. A. role                 B. rule           C. roll                  D. pole

(    ) 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