【单选题】

It’’s hardly news anymore thatAmericans are just too fat.

A、quick look around the mall, the beach or the crowd at any baseball game will leave no room for doubt:our individual weight problems have become a national crisis.Even so, the actual numbers are shocking. Fully two-thirds of U. S. adults are officially overweight, and about half of those have graduated to full-blown obesity.　　It wouldn’’t be such a big deal if the problem were simple aesthetiC、But excess poundage takes a terrible toll on the human body. significantly increasing the risk of heart disease, high blood pressure, diabetes, infertility, and many forms of cancer. The total medical bill for illnesses related to obesity is $117 billion a year-and climbing - and the Journal of theAmerican MedicalAssociation reported that poor diet and physical inactivity could soon overtake tobacco as the leading cause of preventable death in the U. S.　　Why is it happening The obvious, almost trivial answer is that we eat too much high-calorie food and don’’t burn it off with enough exercise. If only we could change those habits, the problem would go away.But clearly it isn’’t that easy.Americans pour scores of billions of dollars every year into weight-loss products and health-club memberships. Food and drug companies spend even more trying to find a magic food or drug that will melt the pounds away. Yet the nation’’s collective waistline just keeps growing.　　It’’s natural to try to find something to blame - fast-food joints or food manufacturers or even ourselves for having too little willpower.But the ultimate reason for obesity may be rooted deep within our genes. Obedient to the inevitable laws of evolution, the human race adapted over millions of years to living in a world of scarcity, where it paid to eat every good-tasting thing in sight when you could find it.　　Although our physiology has stayed pretty much the same for the past 50,000 years or so,we humans have utterly transformed our environment. Over the past century especially, technology has almost completely removed physical exercise from the day-to-day lives of mostAmericans.At the same time, it has filled supermarket shelves with cheap, mass-produced, good-tasting food that is packed with calories.And finally, technology has allowed advertisers to deliver constant, virtually irresistible messages that say "Eat this now" to everyone old enough to watch TV.　　This artificial environment is most pervasive in the U. S. and other industrialized countries, and that’’s exactly where the fat crisis is most acute.The text arranges obesity according to
A、 class.
B． size.
C． grade.
D． rank.