题目内容：
How long can human beings live? Most scientists who study old age think that the human body is ___1___ to live no longer than 120 years. However, 110 years is probably the longest that anyone could hope to live —— if he or she is ___2___ healthy and lucky. Some scientists even say we can live as long as 130 years! Yet, our cells simply cannot continue to reproduce ___3___. They wear out, and as a result, we get old and ___4___ die.Even though we can’t live forever, we are living a ___5___ life than ever before. In 1900, the average American life span (寿命) was only 47 years, but today it is 75 years!
When does old age begin then? Sixty-five may be out-of-date as the ___6___ line between middle age and old age. After all, many older people don’t begin to experience physical and mental ___7___ until after age 75.
People are living longer because more people ___8___ childhood. Before modern medicine changed the laws of nature, many children died of common childhood ___9___. Now that the chances of dying ___10___ are much lower, the chances of living long are much higher due to better diets and health care.
On the whole, our population is getting older. The ___11___ in our population will have lasting effects on our social development and our way of life. Some people fear such changes will be for the worse, while some see ___12___, not disaster, many men and women in their “golden years” are healthy, still active, and young in ___13___ if not in age.
As the society grows old, we need the ___14___ of our older citizens. With long lives ahead of them, they need to ___15___ active and devoted. A．designed
B．selected
C．improved
D．discovered
2.
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