Many of us believe that a person's mind becomes less active as he grows older. But this is not true, according to Dr Jarvik, professor of _ at the University of California. She has studied the mental functioning of aging persons for several years. For example, one of her studies concerns 136 pairs of twins  , who were first examined when they were already 60 years old. As Dr Jarvik continued the study of the twins into their 70s and 80s, their minds did not generally decline   as was expected. However, there was some decline in their psycho-motor speed. This means that it took them longer to finish mental tasks than it used to. But when speed was not a factor, they lost very little intellectual   ability over the years. In general, Dr Jarvik's studies have shown that there is no decline in knowledge or reasoning ability. This is true not only with those in their 30s and 40s, but with those in their 60s and 70s as well. It is true that older people themselves often complain that their memory is not as good as it once was. However, much of what we call "loss of memory" is not that at all. There usually was incomplete learning in the first place. For example, the older person perhaps had trouble hearing, or poor vision, or was trying to learn the new thing at too fast a speed. In the cases where the older person's mind really seems to get worse, it is not necessarily a sign of decline due to old age. Often it is simply a sign of a sad emotional state. This passage is mainly about   _  .
what caused mental decline a new discovery about mental decline the difference between middle-aged and older persons how Dr Jarvik studied mental functioning of the twins
Question: Many of us believe that a person's mind becomes less active as he grows older. But this is not true, according to Dr Jarvik, professor of _ at the University of California. She has studied the mental functioning of aging persons for several years. For example, one of her studies concerns 136 pairs of twins  , who were first examined when they were already 60 years old. As Dr Jarvik continued the study of the twins into their 70s and 80s, their minds did not generally decline   as was expected. However, there was some decline in their psycho-motor speed. This means that it took them longer to finish mental tasks than it used to. But when speed was not a factor, they lost very little intellectual   ability over the years. In general, Dr Jarvik's studies have shown that there is no decline in knowledge or reasoning ability. This is true not only with those in their 30s and 40s, but with those in their 60s and 70s as well. It is true that older people themselves often complain that their memory is not as good as it once was. However, much of what we call "loss of memory" is not that at all. There usually was incomplete learning in the first place. For example, the older person perhaps had trouble hearing, or poor vision, or was trying to learn the new thing at too fast a speed. In the cases where the older person's mind really seems to get worse, it is not necessarily a sign of decline due to old age. Often it is simply a sign of a sad emotional state. This passage is mainly about   _  .
A. what caused mental decline
B. a new discovery about mental decline
C. the difference between middle-aged and older persons
D. how Dr Jarvik studied mental functioning of the twins
Answer: