单项选择题

Citizens of the world’s wealthiest countries may live longer than previously predicted, according to a study sponsored by the National Institute on Aging. By 2050, people in the G-7 nations (Canada, France, Germany, Italy, Japan, the United Kingdom, and the United States) may be living anywhere from 1.3 years to 8 years longer than official estimates now predict. Researchers at Mountain View Research in los Altos, California, examined 50 years of mortality data and found a long-term decline in death rates. That may mean that fewer workers will have to support many more elderly in the future than governments currently expect.

The main theme of the passage is （）in the G-7 nations.

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