据人口学家统计和预测，墨西哥等发展中国家的人口翻一番需20～40年，美国需40～80年，瑞典人口将会相对稳定，德国人口将减少。专家们对人口增减动态的预测依据是
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A．种群数量和密度B．种群年龄结构C．种群性别比例D．出生率和死亡率
相关热点： 人口学家 出生率 死亡率
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据人口学家统计和预测，墨西哥等发展中国家的人口翻一番需20～40年，美国需40～80年，瑞典人口将会相对稳定，德国人口将减少。专家们对人口增减动态的预测依据是
[ ]
A．种群数量和密度B．种群年龄结构C．种群性别比例D．出生率和死亡率
In “Relax, We’ll Be Fine”, columnist David Brooks writes, “the fact is, despite all the problems, America’s future is extremly bright.”
Mr. Brooks acknowledges that “According to recent survey, 60 percent of Americans think the country is heading in the wrong direction. The same percentage believe that the U.S. is in long-term decline. The political system is not functional. A financial crisis looks unavoidable. There are plenty of reasons to be sad.”
But, he writes:
“Demographers（人口学家) predict that over the next 40 years, the U.S. population will increase by an additional 100 million people, to 400 million over all. The population will be determined, hardworking and relatively young. In 2050, only a quarter will be over 60, compared with 31 percent in China and 41 percent in Japan.”
As the rising generation leads an economic recovery, it will also participate in a communal
（of community) one. We are living in a global age of social entrepreneurship（创业精神).
In sum, the U.S. is on the edge of a demographic, economic and social recovery, built on its historic strengths. The U.S. has always been good at destructive change. And it’s always been good at decentralized community-building, too. Surely a country with this much going for it is not going to wait around passively and let a rotten political culture drag it down.
51．In his article Relax, We’ll Be Fine, David Brooks intends to________.
A．encourage Americans to be optimistic about their future
B．give Americans courage to face their financial crisis
C．persuade Americans to live happily and generously
D．inspire Americans to lead an economic recovery
52．There are all the reasons for Americans to be unhappy except____
A．the function-failed political system B．the increasing population
C．the declining economy D．the wrong direction and financial crisis
53．Over the next 40 years, according to David Brooks, Americans will be____
A．more optimistic and humorous B．more pessimistic and stronger
C．more energetic and promising D．more considerate and aggressive
54．In the author’s opinion, America________.
A．has always benefited from wars B．has done well by its political system
C．has contributed a lot to human progress D．has contributed to world’s economy
55．We can learn from the passage that____
A．American political system and culture are backward
B．Americans are not a nation to give in to difficulties
C．Americans are very destructive and aggressive
D．America has been in an economic and social recovery
英国人口学家布莱克(C.P.Blacker)提出的人口转变五阶段模式，早期扩张阶段的特征是()。
A、高出生率
B、低死亡率
C、低增长率
D、高增长率
A、30%
B、40%
C、50%
D、60%
请帮忙给出正确答案和分析，谢谢