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  3. More boys than girls arc born all over the world, but a new study has found that the closer people live to the equator, the smal

More boys than girls arc born all over the world, but a new study has found that the closer people live to the equator, the smal

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问题     More boys than girls arc born all over the world, but a new study has found that the closer people live to the equator, the smaller the difference becomes. No one knows why.
    The skewed sex ratio at birth has been known for more than a hundred years, and researchers have found a large variety of social, economic and biological factors that correlate with it—war, economic stress, age, diet, selective abortion or infanticide and more. Teasing out the contribution of any single cultural or political variable has proved an almost infinitely complicated exercise.
    But latitude is a natural phenomenon, unaffected by cultural or economic factors. To look at its effect, Kristen J. Navara of the University of Georgia used the latitude of the capital city in 202 countries, as well as 10 years of data on sex ratio at birth and annual variations in day length and temperature.
    To estimate socioeconomic status for each country, she used statistics on unemployment and gross national product. She also calculated a political instability index using an analysis of stale failure and conflict published by the Fund for Peace, a research organization that combines 12 social, economic and political indicators to estimate the relative stability of the world’s nations.
    Then Dr. Navara performed a statistical analysis to figure out which variables affect sex ratio. The report appeared online April 1 in Biology Letters.
    The number of boys born was not related to socioeconomic and political factors, but there was a significant correlation between sex ratios skewed in favor of boys and latitude and the climate variables that go with it. African countries produced the lowest sex ratios—50. 7 percent boys and European and Asian countries had the highest with 51. 4 percent.
    The effect of latitude, Dr. Navara found, persisted across wide variations in lifestyle and socioeconomic status. There were large differences in sex ratio between tropical regions within 23 degrees of the equator and the temperate regions 23 to 50 degrees north or south, but no difference between the temperate regions and the subarctic north of 50 degrees. The population of people living south of 50 degrees was too small to be included in the analysis.
    The correlation with latitude was unchanged even after excluding data from Asian and African countries that might have been skewed by abortion or the killing of baby girls. So sex selection by parents before or at birth does not explain the correlation.
The sex ratio at birth______.
选项 A、rests to some extent with the distance to equator
B、is mainly determined by abortion or infanticide
C、correlates only with cultural or economic factors
D、related to every single cultural or political variable
答案A
解析 根据第一段中的“…a new study has found that the closer people live to the equator，thesmaller the differenee becomes”，A应为答案。
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