A、There is not enough financial support. B、It is difficult to measure calories in humans. C、It is harder to control humans’ diet

A study of monkeys over a 20-year period suggests that eating less may extend life and prevent disease. American researchers said they believe their findings could apply to people as well. Richard and his team did the research. Half of the monkeys were permitted to eat as much as they wanted. The other half ate a controlled diet. They were fed 2/3 of the calories they would otherwise choose to eat. The researchers found that 37% of the monkeys that ate as much as they wanted had died of causes related to aging after 20 years. In comparison, only 13% of the monkeys on restricted diets had died. On average, monkeys live about 27 years in captivity, that is, when they live with humans and not in the wild. Richard says researchers do not know why reducing calories increases the length of life. But they believe there is a helpful change in the way the body processes energy in the monkeys that eat fewer calories. Extended life was not the only observable difference between the two groups. The monkeys that ate less had half the amount of heart disease and cancer. The study was paid for by the US National Institute on Aging. Some critics say more research is needed to prove that calorie restriction extends life in monkeys. Richard says his study is not complete and that his team will continue to observe the remaining monkeys throughout their lives. Scientists first studied the effects of calorie restriction on aging in the 1930s. Since then, similar studies have shown life-extending effects of reduced diets on rodents(啮齿动物)and worms. Scientists are currently studying the effects of restricted diets on people. However, this is more difficult because it is harder to control the diets of people and because they live longer than other animals.
20. What can we learn from the study?
21. What needs to be done according to some critics?
22. Why is it hard to study the effects of restricted diets on people?