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.

According to this passage, the status of the aged is lowered by their ______.

A．forced retirement

B．inferior housing

C．longer life span

D．fixed incomes

高频重组菌株简称Hfr侏，其转移F因子的特点是（）

A．F+细菌把整个F因子输送给F－，使F－转变成F+，自己转变成F－

B．F+细菌把其F因子的一条链输送给F－，然后各自复制出其互补链，使F－转变成F+，自己不变

C．F+细菌只能把F因子上的一段DNA输送给F－，但不输送F因子，F－仍然是F－，它自己也不变

D．F+细菌只能把F因子上的一段DNA输送给F－，F+变成是F－，原来的F－转变成F+