题目内容：
共用题干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___1___ are rising all the time. What has brought___2___ these changes?When we look at the life___3___of people 100 years ago,we need to look at the greatest___4___of the time. In the early 20th century,these were the acute and often ___5___ 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 ___6 ___world these diseases are far___ 7 ___today,and in some cases have almost disappeared. A number of ___8 ___have led to this:improvements in sanitation and hygiene,the discovery and use of antibiotics,which ___9___ bacterial diseases much less dangerous,and vaccinations___10___ common diseases. ___11___ ,people's general health has improved with improvements in our general environ-ment: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 ___12___ 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 chro-nic illnesses,such as heart disease and strokes,and those ___13___ by viruses,such as influen- za and AIDS. Of course,cancer is a huge killer as well. In most cases these diseases affect ___14___ people,but there are worrying trends in the developed world with problems such as obesity ___15___ 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 possi- ble to halt their progress.
8._________
A: improvements
B: factors
C: jobs
D: measures
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