单选题 It is generally believed that the greatest damage of old age is the loss of mental faculties. With the near doubling of life expectancy in the past century has come a mixed blessing. A few great thinkers and artists remained productive in their later years—Galileo, Monet, Shaw, Stravinsky, Tolstoy—but even they were not what they bad been in their primes. In sciences, the boom falls sooner still: "A person who has not made his great contribution to science before the age of 30 will never do so," said Einstein.
Imagine if we could transplant old brains into younger bodies: would our minds stay young, or would we be senile teenagers, scaling mountains and skateboarding at 120, but forgetting where we put the car's keys. Is the brain uniquely vulnerable to the ravages of time? Can anything be done?
Indisputable evidence from many studies shows that a higher level of education and greater mental activity throughout life correlated with lower cognitive losses in old age. These benefits apply to all sons of cognitive losses, including those associated with dementia. Some researchers believe that mental application in early life produces complex neural connections that provide a reserve later on; others argue that education merely gives people the means to cope with and compensate for their losses.
K. Warner Shay, a professor of human development and psychology at Pennsylvania State University, has studied age-related change in more than 5,000 people, some for more than 40 years. Comparing earlier with later recruits, Mr. Shay concludes that the rate of mental decline is slowing, a change he attributes to better education, healthier diet, lessened exposure to serious disease, and more mental activity. "You've got to practise," Mr. Shay says. "If you don't solve problems, you no longer can solve problems." Retirement can be particularly hard, he adds, because for many people, work is their most challenging activity. "Retirement is good for people who've had routine jobs—they may find something more stimulating. But it's disadvantageous for people in high-level jobs, who are less likely to find something as stimulating as the job they had."
K. Anders Ericsson, a psychology professor at Florida State University, confirms Mr. Shay's emphasis on the virtue of practice. Initially interested in expert performance like musicians, he found that many geniuses aren't really so different from everyone else—they just practise harder and longer, benefiting from sheer labor, rather than from some special gift. Professional musicians who continue to practise assiduously as they age continue to play well, while amateurs who just play for pleasure show age-related declines.
Mr. Ericsson's studies failed to show significant generalized fitness from mental exercise. If you play tennis, you improve your general fitness, but the greatest improvement is specific to tennis, not to other sports. It's the same with cognitive exercise. You have to look at your life and pick what you want to improve.

单选题 According to the context, "a mixed blessing" (Para. 1) most probably means ______.

单选题 Scientific studies indicate that ______.

单选题 According to the text, retirement can benefit those who ______.

单选题 The example of tennis is mentioned in the last paragraph to suggest that mental exercise ______.

单选题 Throughout this text the author mainly intends to ______.