听力原文： One way that scientists learn about man is by animals, such as mice, rats and monkeys. The scientists in this 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 three 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 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. 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. The last group of mice spends most of the day eating or sleeping. They are not very ac tire. These mice are living longer than the scientists thought: about a year and a hell But they are not healthy. The experiment is still going on. The scientists hope to finish their studies in two years. (33)