资料题：The word"obsolete," in paragraph four means（  ）。

A few years ago it was fashionable to speak of a generation gap, a division between young people and their elders.Parents complained that: children did not show them proper respect andobedience, while children complained that their parents did not understand them at all.What had gone wrong? Why had the generation gap suddenly appeared? Actually, the generation gap has been around for a long time.Many critics ague that it is built into the fabric of our society.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 parents, 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, many 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 division 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 obsolete overnight.The young and the old seen to live in two very different worlds, separated by different skill and abilities.No doubt, the generation gap will continue to be a feature of American life for some time to come.Its causes are rooted in the freedoms and opportunities of our society, and in the rapid pace at which society changes.