Modern industrial society grants little status to old people. In fact, such a society has a system of built-in obsolescence. There is no formal system for continuing our education throughout our life in order to keep up with rapidly changing knowledge. When our education and job skills have grown obsolete, we are treated exactly like those who have never gained an education or job skills and are not encouraged or given the opportunity to begin anew.As a society becomes more highly developed, the overall status of older people diminishes. Improved health technology creates a large pool of old people, who compete for jobs with the young. However, economic technology lowers the demand for workers and creates new jobs for which the skills of the aged are obsolete, forcing older people into retirement. At the same time, young people are being educated in the new technology and are keeping pace with rapid changes in knowledge. Finally, urbanization creates age-segregated neighborhoods. Because the old live on fixed incomes, they must often live in inferior housing. All these factors — retirement, obsolete knowledge and skills, inferior standards of living — lower the status of the aged in society.A century ago, when one could expect to live only to 50 or so, the life span more or less coincided with the occupation and family cycle. But today the average life span allows for fifteen to twenty years of life after these cycles. It appears that our life span is outpacing our usefulness in society.

By "a system of built-in obsolescence" the author means________.

A．no formal systems exist in modern industrial society

B．old people have no status in modern industrial society

C．young people have chances in modern industrial society

D．knowledge changes rapidly in modern industrial society

蒸汽进行表面膜态凝结时，提高凝结换热系数中不正确的方法是（）

A.增加凝结面积；

B.增加蒸汽流速；

C.减少其中不凝性气体含量；

D.机械除液。