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问题 选择题

下列物质中，只含有离子键，不含有共价键的是（　　）

A．H2O

B．KOH

C．NaCl

D．NH4NO3

答案

A、H2O中只含氢氧共价键，故A错误；

B、KOH中钾离子与氢氧根离子之间以离子键结合，氢氧根离子中氢原子和氧原子之间以共价键结合，故B错误；

C、NaCl中钠离子与氯离子以离子键结合，不含共价键，故C正确；

D、NH4NO3中硝酸根离子和铵根离子之间存在离子键，铵根离子中氮原子和氢原子之间存在共价键，故D错误．

故选：C．

单项选择题

我国公务员制度正式建立的时间是（）。

A.1988年

B.1992年

C.1993年

D.1978年
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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.

The underlined word "reside" in Paragraph 1 is closest in meaning to ().

A. fill

B. decide

C. occupy

D. live
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