A．从下列单词中选择适当的词填空，每个词只能用一次. 
Public schools raise cash by selling their students everything from biscuits to soft drinks. This contributes 51 America's childhood overweight problem. The proportion of overweight 52 in the United States more than doubled between 1980 and 2004. For adolescents, that number has more than tripled, In 53 , Congress commissioned a study to examine the snacks sold outside the federally regulated school lunch program. The aim 54 to produce sensible recommendations on what schools should-and should not-offer, The Institute of Medicine responded with 55 sure to upset the average soda-drinking high-schooler. The institute's experts created 56 tiers of snack foods that schools might sell instead of junk. The first contains 57 such as fruits, vegetables, non-fat yogurt and whole-grain chips. These should be 58 to all students any time during the school day. Less nutritious-but not unhealthful-snacks such as 59 potato chips and diet soft drinks are in the second category, 60 high school students only ought to be able to purchase after school. Sure, students can always bring junk food from home._________
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