21. Birds have higher maximum lifespans than mammals (哺乳动物) and are less prone ____ death in the wild.

Afundamentalquestion in ageing research is whether humans and other species possess animmutablelife-span limit. A theoretical study suggested the maximum human lifespan to be around 125 years. The longest-living person whose dates of birth and death were verified to the modern normsof Guinness World Records and the Gerontology Research Group was Jeanne Calment, a French lady who lived to 122. Reduction of infant mortality has accounted for most of the increased average life spanlongevity, but since the 1960s, mortality rates among those over 80 years have decreased by about 1.5% per year. The progress being made in lengthening lifespans and postponingsenescenceis entirely due to medical and public-health efforts, rising standards of living, better education, healthier nutrition and moresalubriouslifestyles.Match the word underlined in the passage with its synonym given in the choicesfundamental （ ） A: ageing B: central C: fixed D: standards E: length