在关系代数中,从两个关系的笛卡尔积中,选取它们属性间满足一定条件的元组的操作,称为A

题型:单项选择题

问题:

在关系代数中,从两个关系的笛卡尔积中,选取它们属性间满足一定条件的元组的操作,称为

A.投影

B.选择

C.自然连接

D.θ连接

考点:计算机等级考试数据库技术国家计算机三级(数据库技术)79
题型:单项选择题

检验一种气体是否是CO2的可靠方法是(  )

A.通入澄清的石灰水,看石灰水是否浑浊

B.将燃着的木条伸入气体中看火焰是否熄灭

C.检验该气体的密度是否比空气密度大

D.通入水中看溶解性

题型:单项选择题

关于流感,下列说法正确的是

A.是由流感病毒引起的急性呼吸道传染病

B.由感冒者的呼吸道分泌物而传染

C.传染性强,传播迅速

D.潜伏期为数小时至4d

E.并发症比较多,老年人和体弱患者易并发肺炎

题型:单项选择题

建筑施工主要产生哪些污染():

A.辐射污染

B.扬尘

C.水污染

题型:单项选择题

It is widely known that any English conversation begins with The Weather.Such a fixation with the weather finds expression in Dr.Johnson’s famous comment that“When two English meet,their first talk is of weather.”Though Johnson’s observation is as accurate now as it was over two hundred years ago,most commentators fail to come up with a convincing explanation for this English weather­speak.

Bill Bryson,for example,concludes that,as the English weather is not at all exciting,the obsession with it can hardly be understood.He argues that“To an outsider,the most striking thing about the English weather is that there is not very much of it.” Simply,the reason is that the unusual and unpredictable weather is almost unknown in the British Isles.

Jeremy Paxman,however,disagrees with Bryson,arguing that the English weather is by nature attractive.Bryson is wrong,he says,because the English preference for the weather has nothing to do with the natural phenomena.“The interest is less in the phenomena themselves,but in uncertainty.”According to him,the weather in England is very changeable and uncertain and it attracts the English as well as the outsider.

Bryson and Paxman stand for common misconceptions about the weather­speak among the English.Both commentators,somehow,are missing the point.The English weather conversation is not really about the weather at all.English weather­speak is a system of signs,which is developed to help the speakers overcome the natural reserve and actually talk to each other.Everyone knows conversations starting with weather­speak are not requests for weather data.Rather,they are routine greetings,conversation starters or the blank “fillers”.In other words,English weather­speak is a means of social bonding.

小题1:The author mentions Dr.Johnson’s comment to show that________.

A.most commentators agree with Dr.Johnson

B.Dr.Johnson is famous for his weather observation

C.the comment was accurate two hundred years ago

D.English conversations usually start with the weather小题2:What does the underlined word “obsession” most probably refer to?

A.A social trend.

B.An emotional state.

C.A historical concept.

D.An unknown phenomenon.小题3:According to the passage,Jeremy Paxman believes that________.

A.Bill Bryson has little knowledge of the weather

B.there is nothing special about the English weather

C.the English weather attracts people to the British Isles

D.English people talk about the weather for its uncertainty小题4:What is the author’s main purpose of writing the passage?

A.To explain what English weather­speak is about.

B.To analyse misconceptions about the English weather.

C.To find fault with both Bill Bryson and Jeremy Paxman.

D.To convince people that the English weather is changeable.

题型:单项选择题

协调分歧利益属于()原则。

A、对事不对人

B、合作

C、互利互惠

D、立场服从利益

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