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It’’s hardly news anymore that Americans 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 the American Medical Association 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 most Americans. 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.
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Solar energy is called a renewable resource because on a human time scale it is essentially inexhaustible, it is expected to last at least 6.5 billion years while the sun completes its life cycle. 46) A potentially renewable resource can be renewed fairly rapidly (hours to several decades) through natural processes, examples of which include forest trees, grassland grasses, wild animals, fresh lake and stream water, groundwater, fresh air, and fertile soil.47) One important potentially renewable resource for us and other species is biological diversity or biodiversity, which consists of the life forms that can best survive variety of conditions currently found on Earth. Kinds of biodiversity include (1) genetic diversity( variety in the genetic makeup among individuals within a single species), (2) species diversity (variety among the species or unique forms of life found in different habitats of the planet), and ( 3 ) ecological diversity (variety of forests, deserts, grasslands, streams, lakes, oceans, wetlands, and other biological communities). 48)This rich variety of genes, species, and biological communities gives us food, wood, fibers, energy, raw materials, industrial chemicals, and medicines―all of which pour hundreds of billions of dollars into the world economy each year. Earth’’s vast list ’’of life forms and biological communities also provides free recycling and purification services and natural pest control.Potentially renewable resources, however, can be exhausted. 49)The highest rate at which a potentially renewable resource can be used indefinitely without reducing its available supply is called its sustainable yield. If a resource’’s natural replacement rate is exceeded, the available supply begins to shrink―a process known as environmental degradation.Several types of environmental degradation can change potentially renewable resources into nonrenewable or unusable resources. In the United States, one-fourth of the groundwater withdrawn each year is not replenished (filled up again). Between 25% and 50% of the world’’s wetlands (55% in the U. S. ) have been drained, built upon, or seriously polluted. Almost half of the world’’s tropical forests have been cleared. Thousands of wildlife species become extinct each year, mostly because of human activities. If habitat destruction continues at present rates, as many as 1.5 million species could disappear over the next 25 years―a drastic loss in vital Earth capital. 50)These examples help explain why most environmental scientists believe that over the next few decades, the danger of degradation and exhaustion is greatest for potentially renewable resources, not nonrenewable resources ( except for petroleum and perhaps a few scarce minerals for which we can’’t find economically and environmentally acceptable substitutes).