阅读理解。 LOS ANGELES-Old people who keep walking a relatively long distance may be less likely to suffer from cognitive (认知的) decline, a new study suggests. 'By walking regularly, and maintaining a little bit of moderate physical activity, you can reduce your likelihood of developing Alzheimer's disease and spare brain tissue,' Kirk I．Erickson, the study's lead author, said.Erickson and his colleagues from the University of Pittsburgh began to establish a link between walking and memory in 1989. According to the report on their study published online Wednesday in Neurology, an official journal of the American Academy of Neurology, researchers tracked the physical activity and cognitive patterns of nearly 300 adults. At the very beginning, all participants, on average 78 years old and about two－thirds being women, were in good cognitive health. The researchers charted how many blocks each person walked in one week. Nine years later, the participants were given a MRI scan to measure their brain size. All of them were deemed to be 'cognitively normal.' But after four more years, test showed a little more than one third of the participants had developed a mild cognitive impairment or dementia. By correlating cognitive health, brain scans and walking patterns, the research team found that being more physically active appeared to lower the risk of developing cognitive impairment. As to how much walking would help prevent cognitive decline, the researchers suggested that walking about six miles, or 9.6 km, per week appears to protect the brain against shrinking in old age. The researchers said the relationship between walking and gray matter volume appears to apply only to people who regularly walk relatively long distances. The more someone walks, the more gray matter tissue the person will have a decade or more down the road in regions of the brain, namely the hippocampus, the inferior frontal gyrus and the supplementary motor area, that are central to cognition. And among the more physically active participants who had retained more gray matter a decade out, the chances of developing cognitive impairment were cut in half, the study said. 1. When did the old people tested in the research begin to show difference in cognitive decline? A. When they were 78 years old. B. When they were 87 years old. C. Between the year 1989 and 1998. D. Between the year 1998 and 2002. 2. What do we know about the gray matter? A. The quantity of it has a decisive influence on the cognitive ability. B. It's something in the muscle that develops from physical activity. C. It's a brain disease that will damage the cognitive ability. D. It's some brain matter that can cure brain diseases. 3. Which of the following is closest in meaning to the underlined word 'shrinking' in Para.9? A. worrying. B. weakening. C. widening. D. wandering. 4. What is TRUE about the research? A. Men and women differ in the decline of cognition. B. The research subjects were required to walk blocks per week. C. The findings suggest the more walking, the better health. D. There were about 200 women involved in the research.