If you could take a large snapshot (照片) of everyone in the United States today, it would contain six minion more females than males. In this country, women outlive men by about seven years. Throughout the modern world, cultures are different, diets are different, ways of life and causes of death are different, but one thing is the same--- women outlive men.
It starts before birth. At conception, male fetuses(胎儿) outnumber female by about 110 to 100; at birth, the rate has already fallen to about 105 boys to every 100 girls. By the age of 30, there are only enough men left to match the number of women. Then women start taking a lead.  Beyond the age of 80, there are nearly twice as many women as men.
“If you look at the top ten or twelve causes of death,” said Deborah Wingard, a disease expert at the University of California at San Diego, “every single one kills more men.” She listed out one sad fate after another – heart disease, lung cancer and so on. Each kills men at roughly twice the rate of that it does women.
More than a century ago American men surpassed and outlived the women. But in the 20th century, women began living longer,  primarily because pregnancy（怀孕) and giving birth to children had become less dangerous. The gap grew steadily. In 1946, for the first time ever in the United States, females outnumbered males.
Parts of the reasons are self-made. Men smoke more, drink more and take more life-threatening chances than women. Men are murdered (usually by other men) three times as often as women are. They commit suicide at a higher rate and have more than twice as many fatal car accidents as women do. Men are likely to be involved in alcohol-related deaths.
But behavior doesn’t explain away the longevity gap.
Today, some scientists studying the gender gap believe that the data point to one conclusion Mother Nature may be partial(偏袒) to women.
Every living thing is assembled according to instructions on its chromosomes (染色体), and humans have 23 pairs of them. But in males, one of these is a weak non-matching pair, expressed by “xy”. The agreement pair in females is “xx”, and its genetic “backup” power is sometimes listed as a clue to woman’s superior restoring force.  If the male’s single “x” chromosome isn’t perfect, it is possible for a serious genetic disorder to appear. Some blood diseases, for instance, are diseases caused by a shortcoming in a single gene on the “x” chromosome. They are far more common in males than females.
What may the underlined word “outlive” in Paragraph1 mean?

A．Become stronger than.	B．Live longer than.
C．Be wiser than.	D．Be born more earlier than.

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