题目内容：
根据材料请回答Exercise Your Brain, Stay YoungIf you want to stay young, sit down and have a good think.This is the research find-ing of a team of Japanese doctors, who say that most of our brains are not getting enough exercising, and as a result, we are aging unnecessarily soon.
Professor Taiju Matruzawa wanted to find out why otherwise healthy farmers in northern Japan appeared losing their ability to think and reason at a relatively early age, and how the process of aging could De slowed down.
With a team of colleagues at Tokyo National University, he set about measuring brain volumes(量,体积)of a thousand people of different ages and varying occupations.
Computer technology enabled the researchers to obtain precise measurements of the volume of the front and side sections of the brain, which relate to intellect and emotion, and determine the human character.(The rear section of the brain, which controls func-tions like eating and breathing, does not contract with age.)
Contraction of front and side parts-as cells die off-was observed in some subjects in their thirties, but it was still not evident in some sixty and seventy-year-olds.
Matruzawa has concluded from his test that there is a simple remedy to the contrac-tion normally associated with age- using the head.
The findings show in general terms that contraction of the brain begins sooner in peo-ple in the country than in the towns.Those least at risk, says Matruzawa, are lawyers, followed by university professors and doctors.White collar workers doing routine work in government offices are, however, as likely to have shrinking.brains as the farm workers, bus drivers and shop assistants.
Matruzawa' s findings show that thinking can prevent the brain from shrinking.Blood must circulate (循环)properly in the head to supply the fresh oxygen that the brain cells need."The best way to maintain a good blood circulation is through using the brain," he says, "Think hard and engage in conversation.Don't rely on pocket calculators."The doctor's test shows that________. A．our brains shrink as we grow older
B．the front section of the brain does not shrink
C．sixty-year-olds have better brain than thirty-year-olds
D．some peoples brains have contracted more than other people's
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