单项选择题

TEXT A
Researchers investigating brain size and mental ability say their work offers evidence that education protects the mind from the brain’ s physical deterioration.
It is known that the brain shrinks as the body ages, but the effects on mental ability are different from person to person. Interestingly, in a study of elderly men and women, those who had more education actually had more brain shrinkage.
"That may seem like bad news," said study author Dr. Edward Coffey, a professor of psychiatry and of neurology at Henry Ford Health System in Detroit. However, he explained, the finding suggests that education allows people to withstand more brain-tissue loss before their mental functioning begins to break down.
The study, published in the July issue of Neurology, is the first to provide biological evidence to support a concept called the "reserve" hypothesis, according to the researchers. In recent years, investigators have developed the idea that people who are more educated have greater cognitive reserves to draw upon as the brain tissue to spare.
Examining brain scans of 320 healthy men and women ages 66 to 90, researchers found that for each year of education the subjects had, there was greater shrinkage of the outer layer of the brain known as the cortex. Yet on tests of cognition and memory, all participants scored in the range indicating normal.
"Everyone has some degree of brain shrinkage," Coffey said. "People lose (on average) 2.5 percent per decade starting at adulthood."
There is, however, a "remarkable range" of shrinkage among people who show no signs no mental decline, Coffey noted. Overall health, he said, accounts for some differences in brain size. Alcohol or drug use, as well as medical conditions such as diabetes and high blood pressure, contribute to brain-tissue loss throughout adulthood.
In the absence of such medical conditions, Coffey said, education level helps explain the range of brain shrinkage exhibited among the mentally-fit elderly. The more-educated can withstand greater loss.
Coffey and colleagues gauged shrinkage of the cortex by measuring the cerebrospinal fluid surrounding the brain. The greater the amount of fluid, the greater the cortical shrinkage.
Controlling for the health factors that contribute to brain injury, the researchers found that education was related to the severity of brain shrinkage. For each year of education from first grade on, subjects had an average of 1.77 milliliters more cerebrospinal fluid around the brain.
For example, Coffey’ s team reported, among subjects of the same sex and similar age and skull size, those with 16 years of education had 8 percent to 10 percent more cerebrospinal fluid compared with those who had four years of schooling.
Of course, achieving a particular education level is not the definitive measure of someone’ s mental capacity. And, said Coffey, education can be "a proxy for many things". More-educated people, he noted, are often less likely to have habits, such as smoking, that harm overall health.
But Coffey said that his team’ s findings suggest that like the body, the brain benefits from exercise. "The question is whether by continuing to exercise the brain we can forestall the effects of (brain shrinkage)," he said. "My hunch is that we can."
According to Coffey, people should strive throughout life to keep their brains alert by exposing them selves to new experiences. Travelling is one way to stimulate the brain, he said; a less adventuresome way is to do crossword puzzles.
"A hot topic down the road," Coffey said, will be whether education even late in life has a protective effect against mental decline.
Just how education might affect brain cells is unknown. In their report, the researchers speculated that in people with more education, certain brain structures deeper than the cortex may stay intact to compensate for cortical shrinkage.
From this passage, we can conclude that______.