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】
D
【答案解析】[解析] 推理题。从题干可知，要求考生推断第一段的隐含意思。从原句“More boys than girls are born all over the world... the smaller 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。即越靠近赤道的地方，男女出生比例(birth rate)变得越低。选项D与第一段的内容相符。
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。本题问除了影响出生率的因素外，Dr. Navara致力于哪些事情。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 of Georgia... at birth 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项和D项不是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。本题问Navara得出结论的依据是什么，从第三段的“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."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—50.7 percent boys—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。题目问哪句话可能解释了性别比例不均衡的原因。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项也被排除。第四段第三句提到“There may be genetic or racial differences that could explain it, but the connection persists over so many varied populations that 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 are just as mysterious as they are in humans，所以D项也被排除。