100 RCs for Practice Questions- 47

The passage given below is followed by a set of questions. Choose the most appropriate answer to each question.
The possibility of untimely death is frightening, but the inevitability of aging and death casts the longest shadow on human life. Humankind's efforts to overcome aging have been impressively persistent. We have, however, not succeeded. By age 80, half of us will die; by age 100, 99 percent; and by about age 115, every one of us will be dead, medical breakthroughs notwithstanding.
During the past few hundred years, the average length of life (expectancy) in modern societies has steadily increased, but the maximum duration of life (span) has not. Centuries ago a few people may have lived to 115; today this maximum remains about the same. All the wonders of medicine, all the advances in public health have not demonstrably increased the maximum duration of life. If aging is a disease, it seems to be incurable.
Technically, we are not really talking about aging, the process of growing older from birth onwards, but senescence, the process of bodily deterioration that occurs at older ages. Senescence is not a single process but is manifested in increased susceptibility to many diseases and a decreasing ability to repair the damage. Death rates in modern developed countries are very low at age 10 to 12, about 0.2 per 1000 children per year. The death rate increases slowly to 1.35 per 1000 at age 30, then increases exponentially, doubling every 8 years. By age 90, the death rate is 169 per 1000. A person aged 100 has only a one-in-three chance of living another year. Every year the mortality curve becomes steeper until eventually we all are gone.
Imagine a world in which all causes of premature death have been eliminated, so that all deaths result from the effects of aging. We would live hearty, healthy lives, until, in a sharp peak of a few years centered at age 85, we would nearly all die. Conversely, imagine a world in which senescence is eliminated, so that death rates do not increase with age but remain throughout life at the level for eighteen-year-olds, that is, about 1 per 1000 per year. Some people would still die at all ages, but half the population would live to age 693, and more than 13 percent would live to age 2000! Even if death rates were much higher, say 10 per 1000, eliminating the effects of senescence would still give a substantial advantage, with some people living to age 300. From an evolutionist point of view, an individual who did not senesce would have, to put it mildly, a substantial reproductive advantage.
This brings us to the mystery. If senescence so devastates our fitness, why hasn't natural selection eliminated it? This possibility seems preposterous only because senescence is such an inescapable part of our experience. Consider, however, the miracle of development: from a single cell with forty-six strands of nucleic acid, a body gradually forms, with each often trillion cells in the right place, making tissues and organs that function together for the good of the whole. Certainly, it should be easier to maintain this body than to form it!
Furthermore, our bodies have remarkable maintenance capacities. Skin and blood cells are replaced every few weeks. Our teeth get replaced once. Damaged liver tissue can be rapidly replaced. Most wounds heal quickly. Broken bones grow back together. Our bodies do have some capacity to repair damage and replace worn-out parts; it is just that this capacity is limited. The body can't maintain itself indefinitely. Why not?