A、8. B、9. C、10. D、11. B题目问的是“根据研究，对于三到六年级的学生来说多少个小时的睡眠是健康的。”由定位句可知，九个小时的睡眠是最好的。故选B。

Weighing too much can damage one’ s health, and obesity is a growing problem for both kids and adults around the world.
    To get a better idea, the researchers interviewed the parents of 785 third graders from around the country. The parents answered questions about how well their kids slept that year. Three years later, the parents answered the same questions. By the sixth grade, 18 percent of kids involved in the study were fat. The scientists found no relationship between weight and the students’ race or gender. It also didn’ t matter how strict their parents were, or whether they were boys or girls. Over the 3 years of the study, the children averaged a healthy 9 hours of sleep a night. Some kids, however, slept a lot more or less than others.
    Q19: How many students the researchers interviewed according to the passage?
    Q20:  According to the study, how many hours is the daily healthy sleep time for the 3rd to 6th graders?
    Q21: According to the passage, what is the most possible factor related to obesity?