请阅读 Passage l,完成第 21~25小题。                                  

请阅读 Passage l,完成第 21~25小题。                                      Passage1   In his book The Tipping Point, Malcolm Gladwell argues that   "social epidemics" are driven in largepart by the actions of a tiny minority of special individuals, often calledinfluentials, who are unusually informed, persuasive, or well-connected. Theidea is intuitively compelling, but it doesn't explain how ideas actuallyspread.    The supposed importance of influentials derives from a plausible-soundingbut largely untested theory called the "two-step flow ofcommunication": Information flows from the media to the influential andfrom them to everyone else. Marketers have embraced the two-step flow becauseit suggests that if they can just find and influence the influentials, those selectedpeople will do most of the work for them. The theory also seems to explain thesudden and unexpected popularity of certain looks, brands, or neighborhoods. Inmany such cases, a cursory search for causes finds that some small group ofpeople was wearing  promoting, or developingwhatever it is before anyone else paid attention. Anecdotal evidence of thiskind fits nicely with the idea that only certain special people can drivetrends.    In their recent work，however，some researchers have come up with the finding that influential havefar less impact on social epidemics than is generally supposed. In fact, theydon 't  seem to be required of all.The researchers ' argument stems from a simple observation aboutsocial influence: With the exception of a few celebrities like OprahWinfrey-whose outsize presence is primarily a function of media, notinterpersonal, influence-even the most influential members of a populationsimply don't interact with that many others. Yet it is precisely these non-celebrityinfluentials who，according to the two-step-flow theory, are supposed to drive socialepidemics, by influencing their friends and colleagues directly.  For a social epidemic to occur, however, eachperson so affected must then influence his or her own acquaintances，who must intum influence theirs, and so on; and just how many others pay attention to eachof these people        has little to do with the initialinfluential. If people in the network just two degrees removed from the initialinfluential prove resistant. for example, the cascade of change won ' tpropagate very far or affect many people.    Building on the basic truth aboutinterpersonal influence, the researchers studied the dynamics of social influenceby conducting thousands of computer simulations of populations, manipulating a numberof variables relating to people's ability to influence others and theirtendency to be influenced.  They foundthat the principal requirement for what is called    "global cascades"---the widespreadpropagation of influence through networks—is the presence not of a few influentials but ,rather, of a critical mass of easilyinfluenced people.By citing the book The Tipping Point, the author intends to_____