单选题. Back in 1975, economists plotted rising life expectancies against countries' wealth, and concluded that wealth itself increases longevity. It seemed self-evident: everything people need to be healthy—from food to medical care—costs money. But soon it emerged that the data didn't always fit that theory. Economic upturns didn't always mean longer lives. In addition, for reasons that weren't clear, a given gain in gross domestic product (GDP) caused increasingly higher gains in life expectancy over time, as though it was becoming cheaper to add years of life. Moreover, in the 1980s researchers found gains in literacy were associated with greater increases in life expectancy than gains in wealth were. Finally, the more educated people in any country tend to live longer than their less educated fellow citizens. But such people also tend to be wealthier, so it has been difficult to untangle which factor is increasing lifespan. Wolfgang Lutz and his colleagues have now done that by compiling average data on GDP per person, lifespans, and years of education from 174 countries, dating from 1970 to 2010. They found that, just as in 1975, wealth correlated with longevity. But the correlation between longevity and years of schooling was closer, with a direct relationship that did not change over time, the way wealth does. When the team put both these factors into the same mathematical model, they found that differences in education closely predicted differences in life expectancy, while changes in wealth barely mattered. Lutz argues that because schooling happens many years before a person has attained their life expectancy, this correlation reflects cause: better education drives longer life. It also tends to lead to more wealth, which is why wealth and longevity are also correlated. But what is important, says Lutz, is that wealth does not seem to be driving longevity, as experts thought—in fact, education is driving both of them. He thinks this is because education permanently improves a person's cognitive abilities, allowing better planning and self-control throughout the rest of their life. This idea is supported by the fact that people who are more intelligent appear to live longer.1. What did economists conclude about longevity in 1975? ______