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  3. Women live longer than men because they are spoiled for choice. That is the startling conclusion of an international group of sc

Women live longer than men because they are spoiled for choice. That is the startling conclusion of an international group of sc

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问题     Women live longer than men because they are spoiled for choice. That is the startling conclusion of an international group of scientists who say they have found the cause of female longevity.
    They say females have two separate cell lines in their bodies, while men have only one. Women in old age can "choose" the more robust (强壮的) of these lines and survive for many more years.
    Why there should be such a persistent cross-cultural gap between the life spans of the sexes has baffled scientists until now. In a paper in the journal Blood, and reported in the current issue of New Scientist, researchers at Odense University in Denmark and Ulleval University Hospital in Norway point to a key difference in the sex-determining chromosomes (染色体) found in the cells of men and women.
    Every cell in a woman’s body has two X chromosomes, one inherited from her mother and the other from her father. In each cell, one of these two X chromosomes is switched off. This means that a woman is a mosaic (混合体) of two cell lines: one derived from her father’s X chromosome, the other from her mother’s. By contrast, every cell in a man’s body has an X and a Y chromosome, neither of which is turned off. He has only one cell line. This difference could be crucial in determining the long-term health of men and women, scientists concluded, and so they decided to examine cells taken from young women.
    They found an even division between their two different lines. However, when they looked at elderly females they found their bodies now favoured one set of cells at the expense of the other line. "This suggests that one of the cell lines is ’better’ than the other," say the researchers. In other words, women’s bodies may select the more vigorous of the two cell lines and allow it to predominate the twilight of their years, giving them a crucial advantage over men, who have no chance of choosing the healthier of two sets of cells.
A great difference between men and women is that ______.
选项 A、women have two chromosomes but men have only one
B、women have two cell lines but men have only one
C、women can choose from two chromosomes but men cannot
D、women can choose from two cells but men cannot
答案B
解析
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