单选题
More boys than girls are born all over the world, but a new study has found that the closer people live to the equator(赤道), the smaller the difference becomes. No one knows why. The imbalanced 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 relate to the sex ratio at birth-war, economic stress, age, diet, selective abortion and more. But latitude(纬度)is a natural phenomenon, unaffected by cultural or economic factors. To look at the effect of latitude, 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. Dr. Navara performed a statistical analysis which showed that there was a significant relation between sex ratios in favor of boys and latitude. African countries produced the lowest sex ratios—50. 7 percent boys—and European and Asian countries had the highest with 51. 4 percent. There are some possible explanations, but none entirely satisfactory. It could be that there is some survival value in producing more girls in warmer regions, but it is unclear what this might be. There may be genetic or racial differences that could explain it, but the connection persists over so many varied populations that this seems unlikely. Mice also produce more male offspring during shorter days or colder weather, but the reasons in these animals are just as mysterious as they are in humans. " There's a possibility that humans might be responding to factors they were programmed to respond to a long time ago-not cultural or socioeconomic, but climate and things like latitude," Dr. Navara said. " What's interesting is that we may be seeing something that connects us with our animal ancestry. "
单选题
What can be learned from the first paragraph?
【正确答案】
A
【答案解析】解析：主旨大意题。从原句“More boys than girls are born all over the world…thesmaller the difference becomes”可知．全世界男人多，女人少，但是离赤道近的地方，男女比例差别变得小了。选项A与第一段的内容相符。第一段没有提及男人习惯于住哪，B可被排除。第一段并没有提及birth rate和birth ratio问题，由此可排除C和D。
单选题
Out of all the influencing factors of birth ratio, Dr. Navara focuses on______.
【正确答案】
C
【答案解析】解析：细节理解题。从第二段的“…and researchers have found a large variety of social，economic and biological factors that relate to the sex ratio at birth——war，economic stress，age，diet，selective abortion and more”可知，与出生婴儿的性别比例有关的社会、经济和生物因素，具体包括：战争、经济压力、年龄、饮食、选择性流产和其他因素。根据关键词Dr．Navara，我们返回到第三段找到相应的定位句：Kristen J．Navara of the University ofGeorgia…atbirth and annual variations in day length and temperature．由此可确定Kristen的关注点是latitude和data。根据以上内容，我们可确定答案为C。A项并未提及，B和D不是Navara关注的内容。
单选题
Dr. Navara reached her conclusion through______.
【正确答案】
A
【答案解析】解析：细节理解题。从第三段的“Kristen J．Navara of the University of Georgia used thelatitude of the capital city in 202 countries，as well as 10 years of data on sex ratio at birth andannual variations in day length and temperature”可知，Navara在研究中引用了data，由此可判断A正确。本段的最后一句：African countries produced the lowest sex ratios一50．7 percentboys—and European and Asian countries had the highest with 51．4 percent就是最佳的证明。而其他三项文章均未提及。
单选题
According to Navara, which of the following probably explains the imbalanced sex ratio?
【正确答案】
C
【答案解析】解析：细节理解题。文章没有提到文化差异对性别比例的影响，由此排除A。人类性别由天生决定的，也不符合原文，故B也被排除。在第四段提到：There may be genetic orracial differences that could explain it，but the connection persists over so many varied populationsthat this seems unlikely，种族不同，其出生率也不同，选项C符合原文这一说法，是正确答案。在第四段的最后提到老鼠的例子，其目的想证明：…but the reasons in these animals arejust as mysterious as they are in humans，所以D也被排除。