问题 单项选择题

在Visual FoxPro中,关于视图的正确叙述是( )。

A.视图不能同数据库表进行连接操作

B.视图是从一个或多个数据库表导出的虚拟表

C.视图与数据库表相同,用来存储数据

D.在视图上不能进行更新操作

答案

参考答案:B

解析: 视图是从一个或多个表中导出的虚拟表,它可使用户从表中提取选定的一组记录,并改变记录的值,然后将更新记录返回源表。它是操作表的一种手段,通过视图可以查询表,也可以更新表。在视图中可以建立索引,也可以与数据库表建立联系。表是用来存储用户数据的,而视图实际上不包含数据,它是虚表。

阅读理解

根据短文内容,从下框的A~F选项中选出能概括每一段主题的最佳选项,选项中有一项为多余项.

A.  The likely reason of global warming

B.Global warming-a world problem

C.The fast increase in recent years

D.What we should do to avoid greenhouse gases producing

E.The earth is becoming warmer

F.Different opinions on the cause of global warming

     1________

     Is the world warming up? According to many reports, the answer is yes. Eleven of the hottest years

since 1850 appeared between 1995 and 2006. Last year, the United Nation's Intergovernment Panel on

Climate Change (IPCC) reported the earth was about 0.75 ? warmer than it was in 1850. While this

doesn't sound like a lot, a small difference in average temperature can make a big difference in climate.

During the last Ice Age, the planet was only about five degrees colder than now.      

     2.________

     It's all about the speed at which temperatures are changing. In the past,temperatures moved up or down step by step. But in the 20th century,  especially since 1976,  temperatures  have  likely risen more quickly

than during any century in the last 1000 years. Warming may bring improved harvests to northern countries such as Canada  and  Russia.  However,  many species won't like  these  conditions.  They begin moving

towards cooler places and cause lots of trouble to human world.

     3.________

     The IPCC reported that human activity is very likely responsible for increasing the greenhouse effect.

Some gases  cause the air  to  take in  heat  energy. Without  the greenhouse effect,  the  earth's average

temperature would be-18? rather than the present comfortable 14.6 ?. The main greenhouse gases are

carbon dioxide, water vapor and some other gases. Burning oil,coal and waste add to the carbon dioxide,

and it has increased 35% since 1850.

     4.________

     Many researchers have found that natural forces alone don't explain the temperature increases over the

last 30 to 40 years. Their study shows that as carbon dioxide goes up, temperature will rise, too. But some scientists don't agree. They think the effect of human produced greenhouse gases is very small compared

to the normal changes that the climate causes.

     5.________

   When asked whether humans are causing the world warming,Carl Wunsch, a professor at Massachusetts Institute of Technology says, "It seems very likely from what I've seen. Is it 100% sure? No. There is

something to do with natural changes." Does that mean we should have a "waitandsee" attitude? No. He

points out we buy house insurance not because we are sure our homes will be on fire, but because it's a

danger we'd rather avoid.

单项选择题