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问题 单项选择题

以下人类活动会对生物多样性造成威胁的是_______．

A．管制狩猎和物种贸易

B．引进外来新物种

C．动物圈养

D．保护栖息地

答案

参考答案：B

解析：生物物种多样化面临威胁的原因包括：生存环境的改变和破坏，掠夺式的开发利用，环境污染，以及外来物种的入侵或引种到缺少天敌的地区．

单项选择题

The fact that most Americans live in urban areas does not mean that they reside in the center of large cities. In fact, more Americans live in the suburbs of large metropolitan areas than in the cities themselves.
The Bureau of the Census regards any area with more than 2,500 people as an urban area, and does not consider boundaries of cities and suburbs.
According to the Bureau, the political boundaries are less significant than the social and economic relationships and the transportation and communication systems that integrate a locale. The term used by the Bureau for an integrated metropolis is an MSA, which stands for Metropolitan Statistical Area. In general, an MSA is any area that contains a city and its surrounding suburbs and has a total population of 50,000 or more.
At the present time, the Bureau reports more than 280 MSAs, which together account for 75 percent of the US population. In addition, the Bureau recognizes 18 megapolises, that is, continuous adjacent metropolitan areas. One of the most obvious megapolises includes a chain of hundreds of cities and suburbs across 10 states on the East Coast from Massachusetts to Virginia, including Boston, New York, and Washington, D.C. In the Eastern Corridor, as it is called, a population of 45 million inhabitants is concentrated. Another megapolis that is growing rapidly is the California coast from San Francisco through Los Angeles to San Diego.

According to the passage, where do most Americans live

A．In the center of cities.

B．In the suburbs surrounding large cities.

C．In rural areas.

D．In small towns.
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问答题 简答题

简述气血的主要功能。
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