passage 4
researchers have established that when people are mental1y engaged, biochemical changes occur in the brain that allow it to act more effectively in cognitive（認(rèn)知的） areas such as attention and memory. this is true regardless of age.
people wi1l be alert (警覺的) and receptive (接受能力強的) if they are faced with information that gets them to think about things they are interested in. and someone with a history of doing more rather than less wil1 go into old age more cognitively sound than someone who has not had an active mind.
many experts are so convinced of the benefits of challenging the brain that they are putting the theory to work in their own lives. "the idea is not necessarily to learn to memorize enormous amounts of information," says james fozard, associate director of the nation-al institute on aging. "most of us don't need that kind of skill. such specific training is of less interest than being ab1e to maintain mental alertness.” fozard and others say they challenge their brains with different mental skills, both because they enjoy them and because they are sure that their range of activities will help the way their brains work.
gene cohen, acting director of the same institute, suggests that people in their old age should engage in mental and physical activities individually as well as in groups. cohen says that we are frequently advised to keep physically active as we age, but older people need to keep mentally active as we1l. those who do are more like1y to maintain 1f heir intellectual abilities and to be generally happier and better adjusted. "the point is, you need to do both", cohen says," intellectual activity actually influences brain cell health and size. ” 

1. peop1e who are cognitively healthy are those______.
a. who can remember 1arge amounts of information
b. who are highly intelligent
c. whose minds are alert and receptive
d. who are good at recognizing different sounds

2. according to fozard’s argument, people can make their brains work more efficiently by______.
a. constantly doing memory work
b. taking part in various mental activities
c. going through specific training
d. making frequent adjustments

3. the findings of james and other scientists in their work ______.
a. remain a theory to be further proved
b. have been challenged by many other experts
c. have been generally accepted
d. are practised by the researchers themselves

4. older people are generally advised to ______.
a. keep fit by going in for physical activities
b. keep mentally active by challenging their brains
c. maintain mental alertness through specific training
d. maintain a balance between individual and group activities

5. what is the passage mainly about ?
a. how biochemical changes occur in the human brain.
b. why people should keep active not only physically but also mentally.
c. how intellectual activities influence brain--cell health.
d. why people should receive special mental training as they age.