单项选择题

To enjoy a longer life has been a dream for mankind since always. With the improvement of health care, nutrition and health knowledge, we began to make dramatic efforts to reduce the effects of the forces that had traditionally shortened human existence.
10 million to 17 million people aged 65 or older made up less than 1 percent of the world’s population in 1900. Survival rates began to climb for infants, children and women of childbearing age, gradually lifting humanity’s average life span. By 2000, 606 million were aged 60 or older, and they made up almost 10 percent of the world’s population. According to the United Nations report World Population Prospects, by 2050 that group could swell to 1.9 billion and constitute one fifth of the world’s projected population.
But it is not all about to live a longer life. And it would not be such a good idea to live your last years in illness and pain. In fact, it would turn into a torture. No, we want to live better, more youthful days while we’re living longer. Diet, exercise and a lucky draw from the gene pool can take us only so far, however. That’s where science comes in.
As medicine tries to find out the means to extend life, culture and its institutions will have to deal with the consequences of success. Age-entitlement programs, such as Social Security, were formed when younger workers far outnumbered retirees, who drew benefits for only a few years; what reforms will longer lives require When savings are used up by parents who may be retired for up to one third of their lives And, equally important, how will we make our extra years emotionally rewarding and rich
Medicine will continue to advance, and, we expect, society and policymakers will have to learn to adapt to the challenges of longevity-both providing it and providing for it-that await us all.
Which one of the following is mentioned as one reason why the old-aged population dramatically increases in the 20th century