A fundamental question in ageing research is whether humans and other species possess an immutable life-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 norms of 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 span longevity, 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 postponing senescence is entirely due to medical and public-health efforts, rising standards of living, better education, healthier nutrition and more salubrious lifestyles.
Match the word underlined in the passage with its synonym given in the choices
senescence （  ）