Read the following passage and answer the questions that follow: In the past…

Read the following passage and answer the questions that follow:

In the past 160 years, life expectancy in the industrialized world has increased at a steady rate of a quarter of a year per year. Initially, this change was driven by the spread of things we often take for granted today: clean water, sewers and waste disposal, better nutrition, vaccines, and antibiotics. By the late 1970s, these innovative changes had reached the point of diminishing returns, yet at about the same time, new technologies began to produce real benefits in the fight against the most fatal remaining threats: cancer, heart disease, and stroke. Even counting the least developed countries, the average person worldwide today can expect to live to nearly 70 years, up from a life expectancy of about 35 years at the beginning of the twentieth century.

While predicting the future is certainly impossible, many progressive thinkers have begun discussing the possibility that at some point in the next twenty years, a child will be born who will live to the today inconceivable age of one hundred and forty years, and further, their children may see a world with no expected "natural" age limit for survival. Barring random accidents or deliberate murder, men and women just two generations from now might potentially live indefinitely. In theory, all that is required is for technology to advance faster than the individual's rate of aging.

However, such optimism must be balanced against the social and physical limits of life. Today's developing technologies at the upper end of the age spectrum will undoubtedly be increasingly expensive and will further widen the gap between the rich and the poor. As lifespans increase, diseases once considered rare exceptions will become more common and massive. Alzheimer's disease, a growing threat to today's elderly, was virtually unknown before the 1950s, primarily because, statistically speaking, most people died before its symptoms could manifest.

Many "progressive thinkers" predict that in the next twenty years, there will be a ________ in life expectancy.