assumption!!

Spokesperson: The major school lunch vendors recently agreed to stop selling high-calorie beverages in elementary and middle schools because studies show that children of ages 7 to 8 who substitute one low-calorie beverage for one high-calorie soft drink in their daily diets will, on average, weigh 20 pounds less than they would have by the time they reach high school. Since only low-calorie beverages will be sold in schools, within six to eight years, we can expect to see a reduction in the percentage of overweight high-school children.

The spokesperson assumes which of the following to be true?
A All low-calorie beverages are better for children than any high-calorie beverages.
B Elementary and middle school students who used to buy high-calorie soft drinks at school will not bring them to school or drink extra high-calorie beverages at home as a substitute.
C The companies will resume selling high-calorie beverages to schools in eight years.
D Those children whose parents are concerned about healthy weight maintenance already limit their children's consumption of soft drinks.
E Many schools depend on the extra revenue generated by sales of high-calorie drinks.

CORRECT ANS IS B

but i dont get is why D is wrng, it can assume that children will not get drink from other source (parents in this case), what if their parent wont limit those children cusumption???

Pls explain,n yeah source is Manhattan!!