Read the following paragraph, find out the topic sentence and write it on the answer sheet. 　Before 1800, no country in the world had an average life expectancy (预期寿命）at birth beyond 40. Today there is not a country that does not Since 1900, more years have been added to human life than in the rest of history combined, initially by reducing child death and lately by stretching life spans. Longevity is one of humanity^ great achievements, yet it is seen as one of society’s great headaches. The problem lies in the increasing dependence of the old on the young. By 2100, the ratio of 65-plus to “working-age” people will triple. As the world grays, growth, tax revenues and work forces will decline while spending on pensions and health care will increase.