单项选择题

Life Expectancy in the Last Hundred Years
A hundred years ago, life expectancy in developed countries was about 47; in the early 21st century, 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 peoplel00 years ago, we need to look at the greatest (54) of the time. In the early 20th century, there were the acute and often (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.