A、Diet. B、Overweight. C、Lack of sleep. D、Sleeping disorder. B短文开篇便提到，肥胖对孩子和成年人来说。都是不断困扰的问题。选B。

Weighing too much can damage your health, and obesity is a growing problem for both kids and adults around the world. Sleep might be one answer to the problem. A new study has found that elementary school students who slept too little were more likely to gain pounds.
    Past studies have shown a link between sleeping less and weighing more, but scientists have had a tough time determining which came first, the chicken or the egg. In other words, it hasn’t been clear whether kids who weigh too much have trouble sleeping, or whether sleeping less leads to weight gain. Both seemed equally possible.
    To get a better idea of which causes which, Lumeng and colleagues interviewed the parents of 785 third graders from around the United States. The parents answered questions about how well their kids slept that year. Three years later, the parents answered the same questions. By sixth grade, 18 percent of kids involved in the study were obese. The scientists found no relationship between weight and the students’ race or gender. It also didn’t matter how strict their parents were. Obesity struck all of these groups equally.
    Instead, sleep seemed to be the key factor. Over the 3 years of the study, the children averaged a healthy 9.5 hours of sleep a night. Some kids, however, slept a lot more—or less—than others. For the sixth graders, every hour of sleep above the 9.5-hour average was linked to a 20 percent lower risk of being obese. Sleep appeared doubly important for the third graders. Every extra hour of sleep they got was linked to a 40 percent drop in obesity by sixth grade.
16. What is the growing problem for both kids and adults worldwide?
17. What confused the scientists in their past studies?
18. What did Lumeng and colleagues do at the beginning of their study?
19. What did Lumeng and colleagues find?