A、One year. B、Three years. C、A year and a half. D、About two years. D在讲第一组老鼠更健康长寿时提到老鼠通常活两年。Mice usually live for twoyears．也就是(D)

One way that scientists learn about man is by studying animals, such as mice, rats, and monkeys. The scientists in a laboratory are experimenting on mice. They are studying the relationship between diet and health. At this time, over one hundred experiments are going on in this laboratory.
    In this experiment, the scientists are studying the relationship between the amount of food the mice eat and their health. The mice are in three groups. All these groups are receiving the same healthy diet. But the amount of food that each group is receiving is different. The first group is eating one cup of food each day, the second group is eating two cups, and the third group of mice is eating three cups.
    After three years, the healthiest group is the one that is only eating one cup of food each day. The mice in this group are thinner than normal mice. But they are more active. Most of the day, they are running, playing with one another, and using the equipment in their cages. Also, they are living longer. Mice usually live for two years. Most of the mice in this group are still alive after three years.
    The second group of mice is normal weight. They are healthy, too. They are active, but not as active as the thinner mice. But they are only living about two years, not the three years or more of the thinner mice.
    The last group of mice is receiving more food than the other two groups. Most of the day, these mice are eating or sleeping. They’re not very active. These mice are living longer than the scientists thought — about a year and a half, but they aren’t as healthy. They’re sick more often than the other two groups.
Questions 22 to 25 are based on the passage you have just heard.
22. What are the scientists in this laboratory studying?
23. Why is the first group the thinnest?
24. How long do normal mice live?
25. What is one possible relationship between the diet of mice and the diet of people?