单项选择题

Scientists can learn about man by studying animals, such as mice, rats and monkeys. The scientists in a laboratory are experimenting on mice and they are studying the relationship between diet and health.
In this experiment, the scientists are studying the relationship between the amount of food the mice can 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 two cups, and the third 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 five for two years, while most of the mice in this group are still alive after three years.
The second group of mice is of 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 is receiving more food than the other two groups. Most of the day, these mice are eating or sleeping. They are not active. These mice are living longer than the scientists though—about a year and a half, but they are not healthy. The are sick more often than the other two groups.

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