According to the passage, which people seem to age sooner than the others

A．they are not clever as our brains
B．they are harmful to our brains
C．we need to use our brains to prevent it from shrinking
D．we could not get enough practice on mathematics

A．a survey of Japanese farmers
B．the study of brain volumes of different people
C．tests performed on a thousand old people
D．the latest development of computer technology