If you feel stressed at school, what can you do to make yourself happier? Some may eat their favorite food or listen to some music. Others may go for a walk or get some exercise. A new study found another great way to lower students' stress—playing with dogs.
Scientists studied 105 students between the ages of eight and nine. 35 trained dogs and their trainers also took part in the test. The children were divided into 3 groups. The students in the first group spent 20 minutes with a dog and its trainer. They watched the trained dogs perform and even played with them. They did this twice a week for 4 weeks. The second group spent the same amount of time (时长) doing exercise to relax. The last group did neither of these things.
During this time, the scientists recorded the students' cortisol levels (皮质醇水平). Cortisol is a stress hormone (激素). The more stressed you feel, the more cortisol you will have in your body. What did the scientists find? Both the second and third groups showed rises in cortisol levels. The first group, however, showed no big increase. And the children in this group had the lowest cortisol levels right after playing with the dogs.
According to the study, students enjoy playing with dogs, and it helpsthem become happier. The stress hormone falls over time this way, which is good for students' health. But here comes the next question: Is 20 minutes of playtime with a dog the best amount of time? Scientists are going to keep working to find out the answer.
根据材料内容选择最佳答案。
B . 
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D . 
