Today people are living to be much older than ever before. Some of the main reasons for this are the improved health care and better nutrition available to everyone.

　　写作思路：

　　21、人们寿命延长的原因在于

　　(1)医学发展，很多病能够得到及时的治疗，人们的寿命延长

　　(2)世界和平，没有大型的战争

　　(3)人们更加注意健康，积极参加体育活动，饮食也比以前合理。

　　参考范文：

　　Medical care is more available to people. Although not everyone can get the best heath care, everyone can get basic heath care and advice. When people are seriously ill, they can go to a public hospital and be taken care of. Years ago, health care wasn't available to everyone. Some people didn't live near a doctor or a hospital, and others couldn't pay for the care they needed. They made do with herbal medicines or folk remedies. Of course, some of these worked, but not for the more serious diseases.

　　The quality of medical care has improved. That’s also a factor in longevity. Doctors know more now about what causes disease and how to care it ,Years ago, doctors only knew about the most basic diseases and cures. Medicine was not very advanced. You could die from something as simple as an infection from a cut. Now we have antibiotics and other medicines to help cure infections.

　　People are also living longer now because of better nutrition. We 're eating better and more healthfully than we used to .That's reduced the number of people with heart disease and cancer. We try to eat low-fat foods and eat more vegetable and fruits ,which are now available year-round.

　　Improved medical care and healthy eating habits has greatly expanded our life spans. What we need to do now is make sure that everyone in the world has these benefits.