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萜类化合物是由 ( )衍生而成，其基本结构可看成是由若干个( )聚合而成。

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In the past fifty years, American society has changed a great deal. Fifty years ago, most Americans lived in small communities. They (1) __ moved from one area to another and knew their neighbors at least by (2) if not by close, personal (3) . Life was so personal in those days that people often (4) _ about it. They said that a person could not even stay home from church on Sunday without the whole town knowing about it. It was difficult to have (5) in a small community like that, but there was usually a sense of security, of belonging, and of community togetherness in such places. Except for church and the local movie theater, there was not much in the way of entertainment. Some people dreamed about moving to the (6) life of the big cities, but most people were happy to live all their lives in the same community.Few people (7) this type of lifelong social interaction or sense of community togetherness now. People often move from neighborhood to neighborhood, city to city, and coast to coast. It is rare to find people who have lives all their lives in one community. Because people move so (8) _, they do not have a chance to get to know their neighbors. Perhaps this is also why Americans tend to have a more casual (9) _ about friendship than people from some other cultures. Americans are (10) __ to leaving friends and making new friends.注意：填字母，不填单词A) attitudeB) interactionC) frequentlyD) nameE) jokedF) rarelyG) experienceH) excitingI) comparativelyJ) correspondenceK) privacyL) cheated M) accustomedN) isolationO) challenge