请阅读Passage l，完成第小题。
Passage 1
Move over Methuselah. Future generations could be living well into their second century and still doing Sudoku, if life expectancy predictions are true. Increasing by two years every decade,they show no signs of flattening out. Average lifespan worldwide is already double what it was 200 years ago. Since the 1980s, experts thought the increase in life expectancy would slow down and then stop, but forecasters have repeatedly been proved wrong.
The reason behind the steady rise in life expectancy is "the decline in the death rate of the elderly", says Professor Tom Kirkwood from Newcastle University. He maintains that our bodies are evolving to maintain and repair themselves better and our genes are investing in this process to put off the damage which will eventually lead to death. As a result, there is no ceiling imposed by the realities of the ageing process. "There is no use-by-date when we age. Ageing is not a fixed biological process," Tom says.
A large study of people aged 85 and over carried out by Professor Kirkwood discovered that there were a remarkable number of people enjoying good health and independence in their late 80s and beyond. With people reaching old age in better shape, it is safe to assume that this is all due to better eating habits, living conditions, education and medicine.
There are still many people who suffer from major health problems, but modern medicine means doctors are better at managing long-term health conditions like diabetes, high blood pressure and heart disease. "We are reaching old age with less accumulative damage than previous generations. We are less damaged," says Professor Kirkwood. Our softer lives and the improvements in nutrition and healthcare have had a direct impact on longevity.
Nearly one-in-five people currently in the UK will live to see their 100th birthday, the Office for National Statistics predicted last year. Life expectancy at birth has c
A.Genes
B.Eating habits
C.Environment
D.Medical condition