Passage One
　　Questions 21 to 25 are based on the following passage.
　　The American population was the tallest in the world from about the American Revolution to World WarⅡ—that's a long time. The U.S.had a very resource-rich environment,with game,fish and wildlife. In fact we have data on disadvantaged people in America,such as slaves. They were obviously among the most mistreated populations in the world, but given the resource abundance, and given the fact that the slave owners needed their work, they had to be fed relatively decently. So the slaves were taller than European peasants. It's no wonder that Europeans were just flooding to America.
　　Americans today are no longer the tallest people in the world.After World WarⅡ,many Western and Northern European countries began to adopt favorable social policies.There is universal health insurance in most of these societies—that,of course,makes a difference in health care.You can also consider income inequality in the U.S., since people who are at the low end of the totem pole(图腾柱)have considerable adversity making ends meet.I suspect the difference in height between Americans and Europeans is due to both diet and health care.
　　Americans today suffer from an additional problem：obesity.If children are too well nourished,then they're not able to grow optimally.There are certain hormones that control the start of the adolescent growth and the start of adolescence.Nutrition is one of the factors,along with genetic and hormonal ones which are associated with the start of puberty(青春期).And if puberty comes too early,then the youth will peak out sooner,and will not become as tall in adulthood as someone who had better nutrition.Also,the overload of carbohydrates and fats in a fast-food diet may hinder the consumption of micronutrients essential to growth.