第6部分：完形填空
阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案。
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 _____l_____ are rising all the time. What has brought _____2_____ these changes? When we look at the life_____3_____of people l00 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_____l0_____common diseases._____11_____, 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_____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 chronic illnesses, such as heart disease and strokes, and those_____13_____by viruses, such as influenza and AIDS l. 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 possible to halt their progress.
