单选题：根据以下资料，回答题。　　If you want to stay young, sit down and have a

根据以下资料，回答题。 　　If you want to stay young, sit down and have a good think．This is the research result of professor Faulkner, who says that most of our brains are not getting enough exercise and as a result, we are ageing unnecessarily soon.
　　Professor Faulkner wanted to find out why healthy farmers in northern Japan appeared to be losing their ability to think and to reason at a relatively early age, and how the process of ageing could be slowed down.
　　He set about measuring brain volumes of a thousand people of different ages and occupations．
　　Computer technology enabled him to obtain precise measurements of the volume of the front and side sections of the brain, which relate to intelligence and emotion, and determine the human character．
　　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.
　　Faulkner concluded from his tests that there is a simple way to slow the contraction—using the head.
　　The findings show that contraction of the brain begins sooner in people in the country than in the towns．Those least at risk, says Faulkner, are lawyers, followed by university professors and doctors．White--collar workers doing routine work are, however, as likely to have shrinking brains as the farm worker, bus driver and shop assistant.
　　Faulkner's findings show that thinking can prevent the brain from shrinking．Blood must circulate properly in the head to supply the fresh oxygen the brain cells need．"The best way to maintain good blood circulation is through using the brain," he says．"Think hard and engage in conversation．Don't rely on pocket calculators." Professor Faulkner wanted to find out__． A． how people's brains shrink
B． the way of making people live longer
C． the size of certain people's brains
D． why certain people aged sooner than others