题目内容：
阅读材料，回答题：Life Expectancy in the Last Hundred Years
A hundred years ago,life expectancy in developed countries was about 47; in the early 21stcentury, men in the United States and the United Kingdom can expect to live to about 74, women to about 80, and these ________(51) are rising all the time. What has brought________ (52)these changes? When we look at the life________ (53) of people 100 years ago, we need to look at the greatest________ (54) of the time. In the early 20th century, these were the acute and of-ten ________ (55)infectious diseases such as smallpox. Many children died very young from these diseases and others, and the weak and elderly were always at risk.
In the________ (56) world these diseases are far________ (57)today, and in some cases
have almost disappeared. A number of________ (58) have led to this: improvements in sanitation and hygiene, the discovery and use of antibiotics, which(59) bacterial diseases much less dangerous, and vaccinations ________(60) common diseases. ________ (61),people's general health has improved with improvements in our general environment: cleaner air,better means of preserving food, better and warmer housing, and better understanding of nutrition.
Genetically, we should all be able to live to about 85 but________ (62)people do live longer today, there are still some big killers around that are preventing us from consistently reaching that age. The problems that affect people today are the more chronic illnesses, such as heart disease and strokes, and those________ (63) by viruses, such as influenza and AIDS. Of course, cancer is a huge killer as well. In most cases these diseases affect________ 64) people, but there are worrying trends in the developed world with problems such as obesity ________ (65)more heart disease and illnesses such as diabetes at younger ages.
The killers today can be classed as. "lifestyle diseases", which means that it may be possible to halt their progress. A．countries
B．lives
C．years
D．ages
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