Which of the following is NOT mentioned in the passage?

(14)Weighing too much can damage one’s health and obesity is a growing problem for both kids and adults around the world. Sleep might be one answer to the cause of the problem.(17-1)A new study has found that elementary school students who slept too little were more likely to gain weight. 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".(15-1)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 cases seemed equally possible.(15-2)To get a better idea of which causes which, 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.(17-2)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. Obesity struck all of these groups equally. Even so, sleep seemed to be the key factor.(16)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.