A、one. B、Two. C、Three. D、Four. C事实细节题。本文每段各阐述厂代沟存在的上个原因，共讲了三个原因。

One important cause of the generation gap is the opportunity that young people have to choose their own lifestyles. In more traditional societies, when children grow up,. they are expected to live in the same area as their parent, to marry people that their parents know and approve of, and often to continue the family occupation. In our society, young people often travel great distances for their education, move out of the family home at an early age, marry--or live with people whom their parents have never met, and choose occupations different from those of their parents.
    In our upwardly mobile society, parents often expect their children to do better than they did: to find better jobs, to make more money, and to do all the things that they were unable to do. Often, however, the ambitions that parents have for their children are another cause of the diversion between them. Often, they discover that they have very little in common with each other.
    Finally, the speed at which changes take place in our society is another cause of the gap between the generations. In a traditional culture, elderly people are valued for their wisdom, but in our society, the knowledge of a lifetime may become dated overnight. The young and the old seem to live in two very different worlds, separated by different skills and abilities.